T cell unresponsiveness to the mitogenic activity of OKT3 antibody results from a deficiency of monocyte Fc gamma receptors for murine IgG2a and inability to cross-link the T3-Ti complex

We recently identified defective monocyte accessory function as the cause of T cell unresponsiveness to the mitogenic activity of OKT3 antibody in cultures of peripheral blood mononuclear cells (PBMC) from five healthy subjects, members of one family. We now report that the underlying abnormality in nonresponders is at the level of monocyte Fc gamma receptors for murine IgG2a. T cell unresponsiveness was not restricted to the signal provided by OKT3 but occurred also for two other anti-T3 antibodies of the IgG2a subclass, in contrast to a normal proliferative response to IgG1 anti-T3 antibodies in one of the OKT3 nonresponders. By using cytofluorography, we found that monocytes from responders but not from nonresponders bound OKT3-FITC to their membrane. The binding could be blocked by mouse IgG2a and by human IgG, but not by mouse IgG1 nor by serum albumin. The data suggest that, through specific Fc gamma receptors for murine IgG2a, monocytes bind the Fc portion of OKT3 during T cell activation. The function of this Fc gamma receptor binding was further studied by culturing PBMC from nonresponders on plates coated with affinity-purified goat anti-mouse IgG antibodies as a substitute for monocyte Fc gamma receptors. The addition of OKT3 to nonresponder PBMC, cultured on such plates, resulted in T cell activation, as evidenced by thymidine incorporation, IL 2 production, and expression of IL 2 receptors. Soluble anti-mouse IgG was not able to substitute for monocyte Fc gamma receptors. The results demonstrate the existence of polymorphism in monocyte Fc gamma receptors for murine IgG2a. They also substantiate that an essential helper function of monocytes in T cell activation by anti-T3 is to provide a matrix for multimeric binding of the Fc portion of the anti-T3 antibodies in order to cross-link T3 molecules.     

Dichotomous effect of monocyte Fc receptor interaction on anti-CD3-induced immunoglobulin synthesis

Monoclonal antibodies against the TCR/CD3 complex are capable of activating T cells which in turn may induce immunoglobulin synthesis in B cells under appropriate conditions. Here we present evidence that distinct immune responses, induced by four commonly used TCR/CD3 mAb (Leu4, OKT3, BMA030, BMA031) were related to the mAb interaction with monocyte Fc receptors for IgG. Depending on their isotype and on the technique by which they were crosslinked, TCR/CD3 mAb induced variable IgM and IgG synthesis in PBMC: If the mAb were crosslinked by monocyte IgG-Fc receptors they induced a high Ig production, while crosslinking the same mAb by plastic-bound goat anti-mouse antibodies (panning) failed to do so. Nevertheless, both crosslinking techniques triggered a strong proliferation and IL-2, IL-4, and IFN gamma lymphokine gene expression. The lack of Ig production under panning conditions was due to an additional IgG-Fc receptor interaction with monocytes: (a) If namely mAb F(ab')2 fragments, or mAb isotypes unable to bind to monocyte Fc receptors (IgG2b, IgG1 in nonresponders) were crosslinked by panning, both a good proliferation as well as Ig production ensued; (b) if TCR/CD3 mAb isotypes which could additionally bind to monocyte Fc receptor (IgG2a) were crosslinked, no Ig production occurred; (c) if mAb F(ab')2 fragments were crosslinked with a second anti-T cell antibody of IgG2a isotype, which could bind to monocyte Fc receptors, Ig synthesis was reduced. Interestingly enough, this diminishing effect, due to monocyte Fc receptor interaction, was only observed if CD4-positive cells were proliferating, but not if CD8-positive cells were activated.     

Failure of OKT3 monoclonal antibody to induce lymphocyte mitogenesis: a familial defect in monocyte helper function

Monoclonal antibodies to the T3 molecule on human T cells have mitogenic activity. Although anti-T3 antibodies of the IgG1 subclass (e.g., UCHT1) induce mitogenesis in lymphocyte cultures from only 60 to 70% of normal donors, antibodies of Ig2a subclass (e.g., OKT3) invariably have been found to be mitogenic in all subjects tested up to the present. This paper describes a family (a mother, six daughters, and one son) in which five members failed to respond mitogenically to OKT3 although the proportion of OKT3-reactive cells in their peripheral blood was normal. Mitogenic responses to PHA, Con A, and PWM were normal. Five members comprising four OKT3 nonresponders were also unresponsive to UCHT1. Unresponsiveness to OKT3 and unresponsiveness to UCHT1 were not absolutely linked to each other, nor were they linked to an HLA haplotype inherited from the mother. Upon stimulation by OKT3, lymphocyte preparations from OKT3-nonresponders failed to produce interleukin 2 (IL 2) and to display IL 2 receptors. OKT3 unresponsiveness was due to defective monocyte help: thus, responsiveness to OKT3 of T cells from an OKT3-nonresponder was restored by the addition of monocytes from an HLA-identical sister who had a normal response to OKT3. Inversely, T cells from the OKT3 responder had no reactivity to OKT3 when cultured in the presence of monocytes from an HLA-identical, OKT3-nonresponsive sister. Unresponsiveness to OKT3 could not be overcome by the addition of phorbol myristate acetate to the cultures. These data on a familial, non-HLA-linked deficiency of monocytes to exert their auxiliary function provide better insight into the mechanism of anti-T3-induced T cell activation.     

Human T cell activation induced by a monoclonal mouse IgG3 anti-CD3 antibody (RIV9) requires binding of the Fc part of the antibody to the monocytic 72-kDa high-affinity Fc receptor (FcRI)

The mitogenic activity of anti-CD3 mouse monoclonal antibodies (mAb) in cultures of human peripheral blood mononuclear cells (PBMC) depends on the ability of the mAb to interact with CD3 molecules on the T cells, and with Fc receptors (FcR) on monocytes. Two types of FcR with distinct specificity for murine (m) IgG subclasses are involved: a 72-kDa receptor (FcRI) binds mIgG2a and a 40-kDa receptor (FcRII) binds mIgG1. In this study we examined the mitogenic activity of mIgG3 anti-CD3 mAb RIV9. In cultures of human PBMC, the mAb induced T cell proliferation and interleukin 2 production. We found that subjects, unresponsive to mIgG2a anti-CD3 (e.g., OKT3), were also RIV9 nonresponders. In contrast, nonresponders to mIgG1 anti-CD3 (e.g., anti-Leu4) had a normal response to RIV9. Our results therefore suggested that anti-CD3 mAb of the mIgG2a and mIgG3 subclass bind to the same monocytic FcR. Human monomeric IgG, which has been shown to bind to FcRI only, blocked T cell proliferation induced by mIgG2a and mIgG3 anti-CD3, but had no effect on T cell proliferation induced by mIgG1 anti-CD3. In contrast, a mAb (IV.3) to FcRII, which blocks ligand binding of the receptor, blocked the mitogenic activity of mIgG1 anti-CD3 antibodies, but had no effect on T cell proliferation induced by mIgG3 anti-CD3 or by mIgG2a anti-CD3. Binding of RIV9 to FcR of responder monocytes could be demonstrated in immunofluorescence. Monocytes from the RIV9 nonresponder subjects however were unable to bind the Fc portion of this antibody. The binding of fluorescein (FITC)-conjugated mIgG3 or FITC-conjugated mIgG2a to responder monocytes could be inhibited by human monomeric IgG and by mIgG2a and mIgG3, but not by the mAb to FcRII. The results demonstrate that mIgG3 binds to FcRI on human monocytes and that this binding is needed for the mitogenic activity of mIgG3 anti-CD3.     

Structural polymorphism of the human monocyte 40 kilodalton Fc receptor for IgG

The 40 kD monocyte Fc receptor for IgG is capable of binding murine IgG1 and of supporting an IgG1 anti-T3 T lymphocyte proliferative response among approximately 80% of Caucasian individuals (responders), whereas the 40 kD Fc receptor on monocytes of the remaining individuals (nonresponders) is incapable of interacting with murine IgG1. By using a monoclonal antibody (mab IV3) that reacts with the 40 kD receptor, we found that the monocyte 40 kD receptors from responder and nonresponder individuals cannot be distinguished by either electrophoretic mobility on SDS-polyacrylamide gels, or by the number of receptors per cell as determined by indirect immunofluorescence. However, isoelectric focussing of the purified radioiodinated 40 kD receptor revealed that the monocyte receptor from all of four nonresponder individuals evaluated has a single distinctive pattern of multiple, regularly spaced bands, whereas the pattern of the 40 kD monocyte receptor from 11 responder individuals is of two sorts. One (seen in four of 11 responders) consists of multiple, regularly spaced bands that are asynchronous with the nonresponder pattern, and the other (seen in seven of 11 responders) consists of multiple bands that correspond in mobility to all of the bands of both of the other two patterns. The incidence of these three patterns suggests that the 40 kD Fc receptor is encoded by a single structural gene with two alleles, both of which are expressed.     

Functional polymorphisms of Fc receptors in human monocyte-mediated cytotoxicity towards erythrocytes induced by murine isotype switch variants

Human monocytes can be triggered to antibody-dependent cell-mediated cytotoxicity (ADCC) by murine antibodies. In this study, a series of H chain isotype switch variant antibodies against glycophorin A on human RBC was used to study the influence of isotype on the induction of ADCC. Furthermore, it was studied whether the functional heterogeneity in responsiveness to IgG1 and IgG2b anti-CD3 antibodies, as found among different donors in T cell proliferation induction experiments, was reflected in ADCC. Whereas IgG2a induced ADCC to the same extent in monocytes from all donors, IgG1 showed a heterogeneous pattern, which corresponded to the heterogeneity in T cell proliferation studies. IgG1 anti-CD3 nonresponder monocytes could, however, be induced to ADCC by IgG1 antiglycophorin, although they needed a much higher antibody density on the target cell than did responder monocytes. IgG2b antiglycophorin at a high density induced ADCC in monocytes from all donors irrespective of responsiveness to IgG2b anti-CD3, whereas IgE and IgA antiglycophorin were barely effective in monocytes from all donors. By specific blocking with mAb, the FcR that were involved in ADCC directed by the various isotypes were characterized. ADCC by IgG2a was predominantly mediated by FcRI and could be specifically enhanced by culturing the monocytes with rIFN-gamma. ADCC by IgG1 was predominantly mediated through FcRII in both anti-CD3 responder and nonresponder monocytes. FcRII was also involved in ADCC by IgG2b, although other receptors seemed to contribute significantly to ADCC. When FcRII or FcRI were blocked, IgG1 and IgG2a could also functionally interact with FcRI and FcRII, respectively, provided that the target cells were sensitized to a high degree. These findings indicate that FcRI and both forms of FcRII can mediate cytotoxicity and that the specificity of human FcR for murine isotypes is relative.     

Direct demonstration of binding of anti-Leu 4 antibody to the 40 kDa Fc receptor on monocytes as a prerequisite for anti-Leu 4-induced T cell mitogenesis

It has previously been demonstrated that about 30% of healthy Caucasian subjects are "nonresponders" in assays of the mitogenic activity of monoclonal mouse IgG1 (mIgG1) anti-CD3 antibodies (e.g., anti-Leu 4 and UCHT-1), and that this unresponsiveness is due to lack of monocyte helper function. In an immunofluorescence assay with fluorescence-activated cell sorter analysis, we studied the binding of phycoerythrin-conjugated anti-Leu 4 to monocytes from responders and nonresponders. Interaction was observed with monocytes from responders only, and was blocked by a murine monoclonal antibody (IV.3) directed to an epitope on the 40-kDa low affinity Fc receptor (FcRII). This indicates that the interaction represents binding of the Fc part of phycoerythrin-conjugated anti-Leu 4 to FcRII on responder monocytes. Indirect immunofluorescence with antibody IV.3 demonstrated, however, that monocytes from both responders and nonresponders express similar levels of FcRII. Thus, nonresponder monocytes apparently express a variant FcRII which is unable to bind the Fc part of mIgG1 antibodies. The anti-FcRII antibody completely blocked anti-Leu 4-induced (but not OKT3 (mIgG2a)-induced) T cell proliferation in cultures of peripheral blood mononuclear cells from responders. The results provide direct evidence that monocytes from anti-Leu 4 responders, but not monocytes from anti-Leu 4 non-responders, are able to bind the Fc part of mIgG1 to FcRII, and that this interaction with FcRII is essential for the mitogenic activity of mIgG1 anti-CD3 antibodies.     

Lymphocyte mitogenesis induced by monoclonal antibodies to the T3 complex. Differential modulation by human IgG

Murine monoclonal antibodies OKT3 (IgG2), 64.1 (IgG2), and Leu 4 (IgG1) react with a common membrane antigen on human T cells and induce potent mitogenesis at concentrations of 1 ng/ml, 10 ng/ml, and 100 ng/ml, respectively. Human serum inhibits the mitogenic effect of antibodies OKT3 and 64.1, but not that of Leu 4. The inhibitor in serum has been identified as immunoglobulin G (IgG) as evidenced by the ability of anti-human IgG-Sepharose affinity columns to retain the inhibitory activity. Various immunoglobulin classes and subclasses obtained from human myelomas differ in their ability to inhibit the OKT3-induced activation. The best inhibition is obtained with the IgG subclasses IgG1 and IgG3, followed by IgG2; IgG4, IgM, and IgA have little if any effect. None of the IgG subclasses inhibit the Leu 4-induced mitogenesis. Indomethacin as well as supernatants containing interleukin 2 (IL-2) can reverse the inhibitory effects of IgG. Prostaglandins (PGE1 and PGE2) inhibit both the OKT3- and Leu 4-induced mitogenesis, thus lacking the selectivity seen with IgG. Since stimulation by the monoclonal antibodies requires the participation of monocytes, an interpretation consistent with the present data is that IgG stimulates monocytes via its Fc portion to release prostaglandins and/or other suppressor factors via an indomethacin-sensitive pathway. The inability of IgG to inhibit Leu 4-induced mitogenesis may therefore relate to an inability of the monocyte subpopulation, which mediates the Leu 4 response, to secrete suppressor factors. These data suggest a potential value of the mitogenic monoclonal antibodies as probes in studying monocyte heterogeneity and T-cell-monocyte interactions.     

Characterization of IgG FcR-mediated proliferation of human T cells induced by mouse and human anti-CD3 monoclonal antibodies. Identification of a functional polymorphism to human IgG2 anti-CD3.

T cell activation induced by mouse anti-CD3 mAb has shown to be dependent on the Ig isotype of these antibodies. A study of isotype dependency of human antibodies, however, seems more relevant to human effector systems, especially in view of the availability of humanized antibodies for clinical applications. We constructed a panel of mouse and mouse/human chimeric anti-CD3 mAb, which differ only in their CH region and hence have identical binding sites and affinity. By using these antibodies, we now studied their ability to induce T cell proliferation in human PBMC and analyzed the classes of IgG FcR involved in these responses. The human (h)IgG1, hIgG3, and hIgG4, as well as mouse (m)IgG2a and mIgG3 anti-CD3 mAb induced an Fc gamma RI (CD64)-dependent T cell proliferation in all donors. Activation with hIgG2 and mIgG1 anti-CD3 mAb was observed to be mediated via the low affinity Fc gamma RII (CD32). It was found that leukocytes in a normal donor population display a functional polymorphism with respect to hIgG2 anti-CD3 responsiveness. This polymorphism was found to be inversely related to the previously defined Fc gamma RII-polymorphism to mIgG1 anti-CD3 mAb. Monocytes expressing the Fc gamma RII mIgG1 low responder (LR) allele support hIgG2 anti-CD3 induced T cell proliferation efficiently, whereas cells homozygous for the Fc gamma RII mIgG1 high responder (HR) allele do not. This observation could be confirmed in T cell activation studies using hFc gamma RIIa-transfected mouse fibroblasts, expressing either the mIgG1 anti-CD3 HR or LR Fc gamma RII-encoding cDNA.     

Antibodies to nonpolymorphic determinants of the Thy-1 molecule inhibit T cell proliferation

The effect of anti-Thy-1 monoclonal antibodies on murine mixed lymphocyte reactions and concanavalin A-induced mitogenesis were investigated. It is demonstrated that rat antibodies against nonpolymorphic determinants of the murine Thy-1 antigen inhibited cell proliferation in the absence of complement. In contrast, antibodies against polymorphic determinants of Thy-1 had no effect on T cell activation. Inhibition of T cell proliferation did not depend on the isotype of the blocking antibody, because both IgM and IgG antibodies against monomorphic determinants were inhibitory, whereas IgM or IgG antibodies against allotypic determinants were inactive. In addition, the blocking activity could not be attributed to the xenogeneic (rat) origin of the antibodies to nonpolymorphic Thy-1 determinants, because rat anti-Thy-1.2 antibodies had no effect on cell activation. Thus, the efficacy of anti-Thy-1 antibodies as T cell inhibitors was determined by the antibody specificity. The suppressive mechanism of anti-Thy-1 antibodies was effective throughout the entire course of mixed lymphocyte reactions. Addition of antibodies at any time point during the first 90 hr of a 120-hr mixed lymphocyte culture resulted in significant suppression of the proliferative response. However, in some cases an early enhancement preceded suppression of the response. The modulation of proliferative responses by anti-Thy-1 did not result from a nonspecific mitogenic effect of the antibodies on T lymphocytes, because no effects were observed when antibodies were added to responder cells alone. These results suggest that the Thy-1 molecule, or a molecule that is located on the cell membrane in close proximity to the Thy-1 antigen, is involved in the activation of T lymphocytes.     

Monoclonal antibodies against T cell differentiation antigens initiate stimulation of monocyte/macrophage oxidative metabolism

Within the first minute after incubation with the mouse anti-human T cell orthoclone monoclonal antibodies OKT3, OKT4, and OKT8, and in the absence of complement, human monocytes generate a burst of highly reactive oxygen metabolites as detected by a luminol-dependent photometric chemiluminescence (CL) assay. The kinetics of the CL responses to these antibodies are identical to that induced by OKM1, the monoclonal antibody to human monocytes and granulocytes. With regard to CL response intensities, OKM1 induces the maximal response and those of OKT3, OKT4, and OKT8 closely reflect the proportion of T cell subsets recognized by these antibodies in peripheral blood. This reaction is also observed when monoclonal antibodies against mouse Lyt surface determinants (Lyt-1 and Lyt-2) and Thy-1 antigen are tested against murine spleen cells. This murine model was further used to investigate the specificity and the mechanism of this reaction. It was demonstrated that the CL response is Lyt antigen specific, occurs upon addition of monoclonal IgG but not IgM antibodies, requires the concomitant presence of CL-producing cells (CLPC) (promonocytes, monocytes, macrophages, and/or granulocytes) and of fully differentiated T cells, and lastly, is mediated via a T cell opsonization process. Selective blockade of bone marrow cell Fc receptors (FcR II) with monoclonal anti-mouse FcR II antibody inhibits the CL response to IgG2b anti-T cell antibody-coated thymocytes and thus strongly suggests that the stimulation of CLPC oxidative metabolism in this model results from the binding of opsonized T cells to plasma membrane Fc receptors. These observations lend additional support to increasing evidence that the initiation of effector functions by monoclonal anti-T cell antibodies may be strictly dependent upon the presence of monocytes and/or macrophages.     

Fc receptors on monocytes cause OKT3-treated lymphocytes to internalize T3 and to secrete IL-2

Monocytes cause OKT3-treated T cells to secrete IL-2 and to lose cell surface T3. We have studied the fate of the "lost" T3. Immunofluorescence microscopy on permeabilized cells showed that monocytes induce T cells to internalize T3. Furthermore, experiments with radioiodinated T cells showed that the internalized T3 was not degraded and exhibited an unaltered polypeptide composition for at least 16 hr. The role of Fc receptors in inducing internalization was also investigated. Fc receptors were depleted from monocytes by allowing the phagocytes to spread on immune complexes. Such depleted monocytes exhibit a fourfold reduction in their ability to promote both internalization of T3 and production of IL-2. A comparable reduction is seen if F(ab')2 fragments of OKT3 were employed in place of intact IgG. Furthermore, monocyte Fc receptors that have been blocked by heat-aggregated human IgG also show much reduced capability for induction of OKT3-mediated T-cell proliferation. We therefore conclude that Fc receptors bind to the Fc domain of OKT3 and thereby cause OKT3-treated T cells to internalize T3 and become activated.     

The monoclonal antibody 41H16 detects the Leu 4 responder form of human Fc gamma RII.

Human FcR for IgG can be divided into three classes (Fc gamma RI, II, and III) based on their structure and reactivity with mAb. Fc gamma RII can be further subdivided into two categories based on functional and biochemical assays. These two Fc gamma RII subtypes were initially recognized by the failure of T cells from 40% of individuals to proliferate in response to mAb Leu 4 (mouse IgG1, anti-CD3), a response that requires the binding of the Fc region of the Leu 4 mAb to Fc gamma RII on monocyte accessory cells. Inas-much as mouse IgG1, does not bind efficiently to the nonresponder form of Fc gamma RII, mAb Leu 4 is unable to induce proliferation in these individuals. IEF data on Fc gamma RII from Leu 4 responder and nonresponder individuals suggested that the structural gene for Fc gamma RII consisted of two allelic forms R (responder) and N (nonresponder) producing the phenotypes RR, RN, and NN. Thus, exclusive expression of the nonresponder allele in monocytes of "nonresponder" individuals, appeared to be responsible for the lack of proliferation observed. In cooperation with the IVth International Conference on Human Leukocyte Differentiation Antigens, we analyzed CDw32 mAb to determine if they could distinguish the responder and nonresponder forms of Fc gamma RII. We report that mAb 41H16 binds preferentially to the responder allotypic form of Fc gamma RII expressed on human monocytes. When quantitative flow cytometry is used to measure the binding of both mAb 41H16 (responder Fc gamma RII) and mAb IV.3 (all myeloid cell Fc gamma RII), we are able to subdivide the responder population into homozygous and heterozygous responders. In addition, mAb 41H16 blocks the binding of mAb IV.3 to monocytes and inhibits proliferation when added to cells before addition of mAb Leu 4. We also show that polymorphonuclear leukocytes and platelets have the same allotypic differences in the binding of 41H16 as do monocytes. However, a subset of lymphocytes (previously shown to be B cells) expresses the 41H16 epitope with no evidence for donor to donor variability.     

Structural polymorphism of the human platelet Fc gamma receptor

A variable T lymphocyte proliferative response to murine IgG1 anti-T3 monoclonal antibodies, in which most North American Caucasians respond whereas a minority do not, is well established. This is most likely the result of a genetic polymorphism manifested by 1) the inability of the monocyte 40-kDa IgG FcR of some individuals to bind murine IgG1, and 2) a distinctive trimorphic pattern on IEF of the monocyte 40-kDa FcR, one form being seen in all individuals who do not respond and another form (or a combination of both forms) being seen in those who do respond. We have evaluated the IEF patterns of the platelet 40-kDa FcR and find that in every individual tested the pattern for platelet FcR correlates with that seen for the monocyte 40-kDa FcR pattern. Furthermore, the platelets of those individuals whose "nonresponder" monocyte 40-kDa FcR did not mediate a murine IgG1 anti-T3 response did not respond with an aggregation reaction to murine IgG1 immune complexes (opsonized E). In contrast, platelets from donors possessing "responder" monocytes displayed positive "aggregation" responses to E coated with murine IgG1 antibody. However, the platelet FcR structural polymorphism described earlier did not correlate with the donor-specific variability in capacity of platelets to respond functionally to aggregated human IgG described in an earlier paper. Rather, the variation in capacity of platelets from individual donors to respond functionally to aggregated human IgG was related to the quantitative expression of platelet FcR. These data indicate that the molecular mechanisms responsible for the platelet 40-kDa FcR structural polymorphism are quite different from the mechanisms governing the variation in quantitative expression of the receptor.     

IL 2 receptor induction on human T lymphocytes: role for IL 2 and monocytes

In this report we studied the requirements for the activation and proliferation of highly purified human T lymphocytes. Purified T cells incubated for 3 days with PHA neither proliferate nor express IL 2 receptors as detected by FACS analysis with the use of anti-Tac antibodies. However, purified T cells incubated with Con A or anti-T3 moAb do not proliferate, albeit 30 to 35% T cells express Tac epitopes. The addition of IL 2, either natural purified or recombinant, resulted in both the appearance of Tac antigen and the proliferation of PHA-activated T cells. Much to our surprise, IL 2 did not induce proliferation of Tac-positive T cells activated by Con A or soluble anti-T3 unless monocytes were added to the cultures. These data suggested that two classes of IL 2 receptors might exist on T cells, one of which was not functionally involved in T cell proliferation. In keeping with this interpretation, we have been able to demonstrate, using a radiolabeled IL 2 binding assay, that anti-T3 moAb induced almost exclusively IL 2 receptors of low affinity (Kd = 30 to 70 X 10(-9) M) and that additional signals, provided by monocytes, are required for the acquisition of high affinity receptors. IL 2 itself can induce high affinity receptors on PHA-stimulated T cells but not on cells activated by Con A or anti-T3. In this latter case the physical presence of monocytes is required and cannot be substituted by IL 1, thus indicating a previously unreported role for monocytes. It is postulated that the contact of monocytes with T cells induces a switch from an inactive low affinity conformation of the IL 2 receptor to a functional high affinity one.     

Cross-linking of both Fc gamma RI and Fc gamma RII induces secretion of tumor necrosis factor by human monocytes, requiring high affinity Fc-Fc gamma R interactions. Functional activation of Fc gamma RII by treatment with proteases or neuraminidase

Cross-linking of Fc gamma R on human monocytes with human IgG has been shown to induce secretion of the inflammatory and immunoregulatory cytokine TNF. In the present study we examined the role of both constitutively expressed monocyte Fc gamma R, the 72-kDa high affinity Fc gamma R (Fc gamma RI), and the 40-kDa low affinity receptor (Fc gamma RII), in the induction of TNF secretion. On the basis of preferential binding of the Fc moiety of murine mAb of different isotype, Fc gamma RI and Fc gamma RII were selectively cross-linked by using either solid-phase murine (m)IgG2a, or solid-phase mIgG1, respectively. On freshly isolated, untreated monocytes only cross-linking of Fc gamma RI with solid-phase mIgG2a induced TNF secretion. The interaction between Fc gamma RII and mIgG1 could be enhanced by treatment of monocytes with proteases or with the desialylating enzyme neuraminidase. After treatment of monocytes with these enzymes, TNF secretion was effectively induced by solid-phase mIgG1, apparently through cross-linking of Fc gamma RII. However, mIgG1-induced TNF secretion differed between protease-treated monocytes from high responder individuals and monocytes from low responder individuals, TNF secretion being considerably less in the latter population. Protease-treated monocytes and mononuclear cells from individuals with an inherited defect in cell membrane expression of Fc gamma RI were induced to secrete TNF by solid-phase human IgG, confirming the capacity of Fc gamma RII to induce TNF secretion. It was not possible to induce TNF secretion by cross-linking Fc gamma RI or Fc gamma RII with anti-Fc gamma R mAb and soluble or solid-phase anti-mIgG, indicating that high affinity Fc-Fc gamma R interactions are necessary to induce release of this cytokine.     

T cell activation via CD2 [T, gp50] molecules: accessory cells are required to trigger T cell activation via CD2-D66 plus CD2-9.6/T11(1) epitopes

Binding monoclonal antibodies (MAb) both to D66 and 9.6/T11(1) epitopes on the CD2 [T,gp50]-defined molecule produces a high level of T cell mitosis. This was observed with a battery of MAb of different isotypes. In contrast, none of the anti-D66 or anti-9.6/T11(1)Ab could trigger T cell proliferation in combination with anti-T11(3). Moreover, all anti-D66-9.6/T11(1) pairs of MAb tested required monocytes to activate T cells which were recruited through their Fc receptors. Variations among normal individuals were observed in the level of response to anti-D66-9.6/T11(1) pairs of Ab, 75% of a population of French Caucasians giving a high response. The level of response of a given individual was determined by his accessory cells. However, the level of response of an individual appeared to be minimally influenced by the isotype of a peculiar anti-D66 or anti-9.6/T11(1) Ab. The addition of exogeneous IL 2 could overcome the removal of accessory cells or the modulation of CD3 molecules. In contrast, IL 2 receptor appearance was not overcome by removal of monocytes. Thus, T cell activation via CD2 seems to be produced by "touching" several definite regions of this molecule which trigger a cascade of events similar to those produced by mitogenic lectins. One can assume that the appropriate conformational changes of the CD2 molecule induced by anti-D66-9.6/T11(1) pairs of Ab are solely produced when they are presented by accessory cells. This leaves open the question of whether accessory cells would also play a more active role.     

The T11 (CD2) molecule is functionally linked to the T3/Ti T cell receptor in the majority of T cells

Most mature human T lymphocytes express both the multichain T3 (CD3)/Ti T cell receptor for antigen (TCR), and the biochemically distinct 55-kDa T11 (CD2) glycoprotein. Stimulating the T11 molecule causes profound T cell proliferation and functional activation in vitro, but the relationship of T11-mediated activation to antigenic stimulation of T lymphocytes in vivo remains unknown. We now present evidence that T11 function is directly linked to TCR components in T3/Ti+ T11+ human T cells. First, we found that stimulating peripheral blood T cells with the mitogenic combination of anti-T11(2) cells with the mitogenic combination of anti-T11(2) plus anti-T11(3) monoclonal antibodies caused the phosphorylation of TCR T3 chains. The predominance of T3-gamma-phosphorylation that occurred in anti-T11(2) plus anti-T11(3)-treated T cells is similar to the pattern previously observed in antigen-stimulated T cell clones. Second, T11 function depended upon concurrent cell-surface expression of the TCR. Thus, when peripheral blood T cells were deprived of cell surface T3/Ti by anti-T3 modulation, anti-T11(2) plus anti-T11(3)-induced mitogenesis and transmembrane signal generation in the form of calcium mobilization were inhibited. The mechanism of TCR-T11 interdependence was investigated in a series of TCR-deficient variants of a T cell lymphoblastoid cell line. T3/Ti negative variants expressed cell surface T11, but anti-T11(2) plus anti-T11(3) failed to cause detectable calcium mobilization. The TCR-deficient variants also failed to express T11(3) activation epitopes after incubation with anti-T11(2) antibodies, suggesting that T11(3) expression is an essential and TCR-dependent intermediate in the T11 activation mechanism in these cells. Taken together, our results suggest that T11 function depends upon cell-surface expression of TCR in many T3/Ti+ T11+ T lymphocytes, and T11-mediated activation is intimately interconnected with TCR activation mechanisms. A model in which stimulating signals delivered via T11 may be a part of antigenic activation of T lymphocytes is presented.     

Proteolysis induces increased binding affinity of the monocyte type II FcR for human IgG

Human monocytes express two types of IgG FcR, Fc gamma RI and Fc gamma RII. These can be assayed by using indicator E sensitized by human IgG (EA-human IgG) or mouse IgG1, (EA-mouse IgG1), respectively. On mouse macrophages, Fc gamma RI is sensitive to trypsin, whereas Fc gamma RII is trypsin resistant. We studied the effects of the proteolytic enzymes pronase and trypsin on human monocyte Fc gamma R. Neither enzyme caused a decrease in rosetting mediated by monocyte Fc gamma RI. Human Fc gamma RII is polymorphic, and monocytes interact either strongly or weakly with mouse IgG1. The interaction of low responder monocytes with mouse IgG1 was dramatically increased (to the level exhibited by high responder monocytes) by protease treatment. The effects of proteases on Fc gamma RII were investigated in more detail by using monocytes from which Fc gamma RI was selectively modulated by using immobilized immune complexes. Proteolysis of such modulated monocytes induced an increased interaction with EA-human IgG. Fc gamma RII appears to mediate this interaction. This conclusion is supported by the observation that after proteolysis, the Fc gamma RII-mediated binding of EA-mouse IgG1 becomes susceptible to inhibition by (monomeric) human IgG. To quantify the effect of proteolytic enzymes on Fc gamma RII, we performed binding studies with cell line K562, that expresses only Fc gamma RII. A significant increase in Ka of Fc gamma RII for dimeric human IgG complexes was observed when K562 cells were treated with protease. To elucidate the mechanism of this enhancement of Ka by proteolysis, we performed immunoprecipitation studies. Neither m.w., nor IEF pattern of Fc gamma RII were influenced by proteolysis. Moreover, the expression of Fc gamma RII was not affected by proteolysis as evidenced by immunofluorescence studies and Scatchard analysis, and neither were Fc gamma RI or Fc gamma RIII induced. We conclude that proteolysis increases the affinity of Fc gamma RII for human IgG, and speculate that such a proteolysis-induced change may also occur in vivo, e.g., at inflammatory sites.     

Inhibition of the high affinity Fc receptor (Fc gamma RI) on human monocytes by porphyrin photosensitization is highly specific and mediated by the generation of superoxide radicals

p72 high affinity receptors (Fc gamma RI) for the Fc portion of IgG molecules on human peripheral blood monocytes mediate a variety of beneficial functions, but also have deleterious effects in certain clinical situations. In the present study, the photosensitizing porphyrins hematoporphyrin derivative and dihematoporphyrin ether (DHE), which are known to preferentially affect the cell membrane, were found to significantly inhibit binding of mouse IgG2a antibodies to the ligand binding site of Fc gamma RI on human peripheral blood monocytes and the U937 human monocytic cell line. Fc gamma RI receptors could be identified with a monoclonal antibody which recognizes an epitope distinct from the ligand binding site, indicating that photosensitization induced a structural alteration rather than loss of the receptor molecule from the cell surface. The effect of DHE and light appeared to be highly specific, since binding of monoclonal antibodies to other surface structures was not decreased. DHE plus light-induced modulation of Fc gamma RI was found to be mediated by superoxide anions, since addition of a mimic of superoxide dismutase restored both binding of mouse IgG2a to Fc gamma RI as well as human monocyte accessory cell function. These studies identify porphyrin photosensitization as a unique mechanism by which to selectively down-regulate Fc gamma RI-mediated functions.