The role of hapten-reactive T lymphocytes in the induction of autoimmunity in mice. II. Termination of self-tolerance to erythrocytes by immunization with hapten-isologous erythrocytes.

When mice which had been primed with hapten-isologous protein conjugate (PAG-MGG) were challenged with PAB-conjugated isologous mouse erythrocytes (MRBC), they developed Coombs positivity and anemia. However, when mice primed with hapten-heterologous protein conjugate (PAG-HGG) were challenged with PAB-MRBC, neither Coombs positivity nor anemia developed.Since it was demonstrated that PAB-reactive helper T cells were generated by immunization with PAB-MGG but not with PAB-HGG, PAB-reactive helper T cells were considered to play a very crucial role in the induction of autoantibody. These results, as a model for autoantibody production in mice, were discussed on the basis of cellular cooperation mediated by a hapten-mechanism, and are consistent with the hypothesis proposed by Weigle for the mechanism of termination of self-tolerance.     

Induction of anti-hapten antibody response by hapten-isologous carrier conjugate. I. Development of hapten-reactive helper cells by hapten-isologous carrier

Anti-hapten antibody production was elicited by the immunization of hapten-isologous carrier conjugate (DNP-MγG) in mice. The spleen and lymph node cells taken from those primed mice were effectively stimulated with hapten-heterologous carrier conjugates (DNP-KLH and DNP-BαA) as well as hapten-homologous carrier conjugate (DNP-MγG) when transferred into X-irradiated recipient mice. The reactivity of DNP-MγG-primed cells to DNP-heterologous carrier conjugates was not due to the mutual crossreactivity of the carrier with MγG on cellular level, since the spleen and lymph node cells primed with DNP-KLH or DNP-BαA could only be stimulated with corresponding hapten-homologous carrier conjugate. The responsiveness of DNP-MγG-primed cells to hapten-heterologous carrier conjugates was due to the result that hapten-reactive helper cells were developed by the immunization of hapten-isologous carrier and these cells cooperated with hapten-specific B cells.The helper activity of the hapten-isologous carrier-primed cells was resistant to 600-R X-irradiation in vitro and sensitive to in vivo ATS treatment. This suggests that the helper activity induced by hapten-isologous carrier is of T cell origin. The helper activity of hapten-isologous carrier-primed cells was also developed by the immunization of PAB-MγG, and clear cooperative interaction between PAB-MγG-primed cells and DNP-specific B cells was demonstrated through DNP-MγG-PAB.The possible mechanism of helper cell development induced by the immunization of hapten-isologous carrier conjugate was discussed in light of the hapten specificity of helper activity.     

Induction of anti-hapten antibody response by hapten-isologous carrier conjugate. II. Specificity of hapten-reactive helper T cells.

Anti-hapten antibody production was elicited by the immunization of hapten-isologous carrier conjugate (PAB-MGG) in mice. Spleen and lymph node cells taken from these primed mice could demonstrate their helper activity for anti-DNP antibody production when transferred intravenously into 600R X-irradiated recipient mice along with DNP-primed B cells and the double hapten conjugated carrier, DNP-MGG-PAB. Isologous carrier (MGG)-primed cells could not demonstrate this helper activity. Accordingly, helper cells reactive for a haptenic group are considered to develop by the immunization of hapten-isologous carrier conjugate. Hapten-reactive helper activity was also induced by the immunization of other hapten-isologous carrier conjugates, e.g., MAB-MGG, PABS-MGG or PAB-MSA. These hapten-reactive helper cells were T lymphocytes, as the helper activity of PAB-MGG-primed cells was completely abolished by in vivo ATS-treatment. Helper activity of PAB-MGG-primed cells for DNP-primed B cells was also demonstrated through the double hapten conjugated heterologous carrier DNP-HGG-PAB to be the same as with DNP-MGG-PAB, but weakly through DNP-KLH-PAB. As HGG but not KLH resembles MGG in composition, almost all hapten-reactive helper T cells can be considered to recognize not only haptenic groups but also physicochemical properties of the hapten-conjugated carrier site. However, these helper T cells could discriminate structural differences among related haptenic groups, because PAB-MGG-primed cells clearly responded to DNP-MGG-PAB to demonstrate their helper activity for DNP-primed B cells, but responded only weakly to DNP-MGG-PABS or DNP-MGG-MAB. When the specificity restrictions of T and B cells to the same haptenic group were compared by responsiveness measured after the antigenic stimulation (B cell function by anti-hapten antibody production and T cell function by helper activity), differences were noted, as PAB-MGG-primed T cells could respond not only to DNP-MGG-PAB but also fairly well to DNP-MGG-MAB to demonstrate their helper activity, but PAB-MGG-primed B cells responded to only PAB-MGG. Thus, hapten specificity appears to be much more restricted for B cells than T cells. The difference of this responsivity between B cells and helper T cells was thought to derive from the specificity difference of B cell and helper T cell receptors rather than from any sensitivity differences of the experimental procedure. The differences in the specificity restrictions of receptors of B and helper T cells were discussed in the light of hapten-specificity.     

Augmentation of auto-anti-idiotypic antibody production by hapten-reactive helper T lymphocytes

Augmented auto-anti-idiotypic antibody production was effectively achieved by immunization of mice with haptenated myeloma protein in the presence of hapten-reactive helper T lymphocytes. Hapten-reactive helper T-lymphocyte activities were raised in BALB/c mice by immunization with para-azobenzoate (PAB)-derived mouse gamma globulin (MGG) prepared by amidination reaction (PAB_im-MGG). Helper T cell activity was effectively enhanced by pretreatment of mice with a PAB-derived nonimmunogenic copolymer of D-glutamic acid and D-lysine (D-GL) (PAB-D-GL) 3 days before priming with PAB_im-MGG; PAB-D-GL is a potent tolerogen of both PAB-specific suppressor T lymphocytes and PAB-specific B cells. After induction of these enhanced PAB-reactive helper T lymphocytes, mice were immunized with PAB-coupled TEPC-15 myeloma protein (PAB_im-T-15), which was also prepared by amidination reaction. Mice immunized in this way manifested strikingly enhanced titers of auto-anti-idiotypic antibodies, specific for the T-15 idiotype, as compared to control mice which had not been preimmunized with PAB_im-MGG. The ability of PAB_im-MGG preimmunization to facilitate auto-anti-idiotypic antibody production was due to the activity of PAB-reactive helper T cells since PAB-specific B cells had been abolished by prior treatment with PAB-D-GL. The implications of this model for future studies on immunological engineering the analysis of idiotype network phenomena are discussed.     

Interference with tolerance induction in vivo by inhibitors of prostaglandin synthesis

Prostaglandins (PG) have been implicated as modulators of both humoral and cellular immune responses. In order to evaluate a possible role for PG in tolerance, the effect of inhibitors of prostaglandin synthesis on tolerance induction and circumvention has been investigated. Injection of deaggregated human gamma-globulin (DHGG) into A/J mice leads to unresponsiveness to a subsequent challenge with immunogenic aggregated human gamma-globulin (AHGG). Administration of indomethacin (IM) or acetylsalicylic acid (ASA) shortly before and after DHGG injection prevents tolerance induction. PGE2 reverses the tolerance overriding effect provided by IM. IM is not able to overcome unresponsiveness when given 10 and 20 days after tolerance induction, at a time point when both T and B lymphocytes are tolerant. As previously shown, lipopolysaccharide (LPS) both inhibits the induction of tolerance to HGG and circumvents tolerant T helper cells late in tolerance when competent B cells are present. In contrast, IM is unable to circumvent T-helper cell tolerance when given at Day 60 after tolerogen, when B cells (but not T cells) are responsive. Furthermore, LPS acts as an adjuvant, B-cell mitogens, inducer of polyclonal Ig secretion, and primes mice when given with tolerogen, while IM has none of these properties. These results indicate a difference between the effects of IM and LPS on tolerance and a possible role of PG in DHGG-mediated tolerance induction.     

Tolerance induced by TNP-derivatized syngeneic erythrocytes: evidence for cooperation between hapten-specific T and hapten-specific B lymphocytes in the immune response.

Tolerance to the DNP haptenic determinant was induced with a single i.v. injection of trinitrophenylated syngeneic red blood cells. The tolerant state lasted 1 month and was stable on transfer to irradiated thymectomized syngeneic recipients. Suppressor activity was found soon after injection of tolerogen but was lost before the termination of tolerance. The unresponsive state could be reversed by adding normal thymus cells to tolerant spleen cells but not by normal bone marrow cells. LPS when given with immunogen restored the normal immune response in tolerant mice. Thus the injection of TNP-MRBC induced partial immune unresponsiveness which was characterized by the induction of T cell suppressor activity and by a hapten-specific helper T cells tolerance. Finally, these studies suggest a cooperative interaction between DNP-specific T lymphocytes and DNP-specific B lymphocytes in the immune response to DNP-BGG.     

B cell heterogeneity—Difference in the size of B lymphocytes responding to T dependent and T independent antigens

Spleen cells from either normal (nonimmunized) mice or mice preimmunized with TNP KLH were depleted of T cells by treatment with a heterologous anti θ serum and complement. Fractionation of these B cells by velocity sedimentation followed by challenge with either a T independent antigen (DNP POL) or a T dependent antigen (TNP KLH), the latter being performed in the presence of additional helper T cells, revealed apparent size difference between B cells responding to the two antigens. This difference, while most marked with preimmunized B cells, was also apparent with normal B cells from the spleen or bone marrow, but not from the lymph node. Similar data were observed with other T dependent and T independent antigens. The differences in the sedimentation profile of splenic B cells for T dependent and T independent antigens did not seem to be due to a difference in the kinetics of appearance of antibody upon stimulation with these antigens, though large B cells did seem to give rise to antibody producing cells at later times than small B cells.     

Active transformation of tolerogenic to immunogenic signals in T and B cells by 8-bromoguanosine

The injection of deaggregated human gamma-globulin (DHGG) into A/J mice results in the establishment of a state of unresponsiveness to subsequent challenge with immunogenic aggregated human gamma-globulin (AHGG). Administration of the B cell activator 8-bromoguanosine (8BrGuo) 3 hr after administration of DHGG converts the tolerogen to an immunogen and results in an antibody response of even greater magnitude than the primary response elicited by AHGG alone. Adoptive transfer studies with separated populations of T and B cells demonstrated that although transformation of the tolerogenic signal to an immunogenic signal involves effects of 8BrGuo on both T cells and B cells, the major effect appears to be activation of antigen-specific T cells that would otherwise become tolerant. Modulation of T cell tolerance could conceivably be mediated either by direct or indirect mechanisms. Interestingly, optimal responsiveness of B cells from animals treated with DHGG and 8BrGuo is not a T cell-independent event, but requires antigen-reactive T cells. 8BrGuo is not able to override unresponsiveness when given 10 to 20 days after tolerance induction, at a time point when both T and B lymphocytes are tolerant. However, when given at day 60, when T cells (but not B cells) remain tolerant to this antigen, the nucleoside is able to terminate the tolerant state prematurely, possibly by providing an alternate T helper-like signal directly to B cells or by recruiting nonspecific functional T helper cells.     

Hapten-specific tolerance induced by hapten conjugates of D-glutamic acid, D-lysine (D-Gl) or isologous gamma-globulin: evidence for central B cell tolerance in the presence of carrier-primed helper T cells.

A comparison has been made of the well known hapten-specific tolerance systems induced, respectively, by hapten-D-GL or hapten-isologous gamma-globulin conjugates. The principal question addressed in this study concerned the comparative maintenance of B cell tolerance, induced by one or the other method, after adoptive transfer into carrier-primed, irradiated recipient animals and, in addition, what role, if any, might be played by T lymphocytes in the tolerant donor cell population in maintaining such tolerance. The results clearly show that insofar as the hapten-specific B cell is concerned, no obvious difference exists in the capacity to maintain tolerance adoptive transfer between the hapten-D-GL and hapten-isologous gamma-globulin systems; such cells remained tolerant even in the presence of excess helper T cell activity. Moreover, under the conditions employed, depletion of T lymphocytes from the tolerant donor cell population did not affect the maintenance of hapten-specific B cell tolerance after adoptive transfer to irradiated recipients.     

Establishment of unresponsiveness in primed B lymphocytes in vivo

As an approach to examine the influence of the state of cellular activation on the ability to tolerize B cells, the induction of unresponsiveness in human gamma-globulin-(HGG) primed B lymphocytes was studied in an adoptive transfer system. In contrast to transferred normal spleen cells, spleen cells from HGG-primed mice are not readily rendered unresponsive when exposed to the tolerogen, deaggregated HGG (DHGG), in irradiated recipients. A kinetic study showed that unfractionated primed spleen cells do not respond to an antigenic challenge given between 6 and 10 days after cell transfer and injection of DHGG, indicating that they are transiently depressed. In contrast, isolated primed B cells are tolerized when transferred to recipients and treated with DHGG in the absence of T cells. Furthermore, primed B cells exposed to tolerogen in the recipients do not recover the ability to respond to HGG either after a secondary challenge with AHGG given up to 14 days after transfer, or after 2 consecutive challenges given on days 14 and 24 after transfer. The presence of primed T cells at the time of tolerization interferes with the induction of unresponsiveness in these primed B cells. These studies suggest that the presence of primed T cells is responsible for the inability to tolerize unfractionated primed spleen cells populations and that primed B cells themselves are not intrinsically resistant to the induction of unresponsiveness.     

Functional clonal deletion and suppression as complementary mechanisms operative in adult hapten-induced cytotoxic T cell tolerance

Immunologic tolerance to the hapten TNP was induced in adult mice through the i.v. injection of reactive TNBS. To probe the cellular basis of the tolerant state, splenic cytotoxic T lymphocyte precursors (CTL-P) were stimulated in vitro with haptenated, x-irradiated syngeneic spleen cells in the presence or absence of exogenously added growth factors derived from Concanavalin A-stimulated spleen cell conditioned medium (CAS). The cultures were either conventional bulk cultures or limit dilution cloning cultures. For the latter, cytotoxicity was assessed through a semi-automated, radioautographic 111In-release assay. Suppressive potential was assessed by mixing spleen cells from tolerant mice with normal spleen cells before culture. In the absence of CAS, bulk cultures showed profound tolerance, and suppressive capacity was clearly evident. Suppression was dependent on the presence of TNP-self during culture and affected the generation of CTL from CTL-P and not the effector function of CTL. Cyclophosphamide treatment did not prevent tolerance induction. In the presence of CAS, bulk cultures still showed marked tolerance, but mixing experiments now yielded no evidence of suppression. As documented previously, limit dilution cultures of tolerant spleen cells in the presence of CAS showed a functional clonal deletion of hapten-specific CTL-P. In the absence of CAS, limit dilution cultures became dependent on helper T cells as the limiting element. Tolerant populations showed a diminution of activatable helper T lymphocyte precursors (HTL-P), which may have been due to a functional clonal deletion of HTL-P and/or a concomitant activation of suppressor T cells. Adoptive transfer studies showed that cells from tolerant mice did not detectably influence the number of hapten-specific CTL-P in the spleens of host animals. Taken together, the results suggest that both functional clonal deletion of CTL-P and suppression of HTL-P contribute to the tolerant state induced.     

Mechanisms in immune tolerance. I. A specific block of immunological memory in HSA-tolerant mice

Evidence is provided indicating that the transient immune response detected during the induction of tolerance to HSA in mice, results in immunological memory which persists in an immunosuppressed state in the tolerant animals. The mechanism which blocks this memory is antigen-specific and can be selectively inactivated by total body irradiation in the range of 650–900 R. Consequently tolerant mice, irradiated within this range and restored with normal syngeneic spleen cells, respond better to the tolerizing antigens than do similarly treated normal mice. The tolerizing block can be transmitted from “tolerant” to normal spleen cells when a mixture of both is transferred to normal irradiated recipients. Since similar inhibitory activity is also demonstrated by fresh sera of tolerant mice, it is suggested that the blocking mechanism depends on inhibitory cells which act via a humoral inhibitor.     

Studies on the kinetics of hemopoietic stem cells and immune responses to the hapten-carrier conjugate.

The correlation between the kinetics of hemopoietic stem cells and immune responses to the hapten-carrier conjugate was investigated. The numbers of both pluripotent stem cells (CFU-S) and myeloid stem cells (CFU-C) in the spleen from mice immunized with the hapten-carrier conjugate were significantly greater than those of the control and the activity of colony-stimulating factor (CSF) in the serum of these mice was markedly elevated. The supernatant of short-term incubation of splenic T lymphocytes from these mice, when stimulated with carrier protein, had high levels of both activities of CSF and helper T cell factors. The study by gel chromatography showed that these factors are similar m.w. substances of 35,000 to 45,000 daltons. But analysis by ion-exchange chromatography demonstrated that they do not have identical biochemical properties. The present studies suggest that biologically active factors produced by T cells stimulated with carrier protein may induce the enhancing effect on the proliferation and differentiation of hemopoietic stem cells and immune responses to the hapten-carrier conjugate.     

Induction of immunologic tolerance in nursing neonates by absorption of tolerogen from colostrum.

Deaggregatedhuman gamma-globulin (DHGG) injected into female mice within 24 hr after delivery of a litter enters the colostrum and is absorbed intact through the intestine by nursing neonates. This absorbed HGG was present in the neonatal circulation at concentrations of 0.3 to 0.6 mg/ml of serum under the experimental conditions used. This absorption of HGG by the nursing neonate resulted in a complete, specific, tolerant state to HGG. This tolerant state was stable upon adoptive cell transfer and could not be abrogated by transfer of normal syngeneic spleen cells.     

Specific, transient suppression of the immune response by HGG tolerant spleen cells. II. Effector cells and target cells.

In a previous report, it was shown that spleen cells from mice made tolerant to human gamma-globulin (HGG)5 could specifically inhibit the immune response of normal spleen cells after adoptive transfer to lethally irradiated recipients. However, that report also showed that the suppressive activity was only transiently associated with tolerant spleen cell populations. It was concluded from those experiments that while suppressive activity could be demonstrated in tolerant spleen cells under certain conditions, such activity was not obligatory for the maintainance of the tolerant state. The experiments presented here were performed to determine the nature of the effector cell(s) and the target cell(s) involved in this system of suppression of the immune response. Treatment of cells from tolerant animals with anti-thymocyte serum and complement to remove thymus-derived (T) cells completely abrogated suppresive activity. Removal of adherent cells from tolerant spleen cells by passage over glass wool columns resulted in partial loss of the suppression. The inhibitory activity of the suppressor cells was resistant to 900 R irradiation regardless of whether the tolerant spleen cells were irradiated before or after adoptive transfer. The cellular target(s) for the supprssor cells was examined by using lipopolysaccharide (LPS) as an alternative source of helper activity for the response to HGG. LPS, injected at the time of the initial antigenic challenge of mice that had been reconstituted with tolerant and normal spleen cells, prevented the expression of suppression against bone marrow-derived (B) cells. However, when LPS was presented only at the time of secondary antigenic challenge, it was unable to overcome suppression of the immune response of reconstituted recipients. Thus, LPS could produce a state where the B cells were resistant to suppression, but LPS could not rescue the responsiveness of B cells once the cells in the reconstituted recipient had been suppressed. In addition, the immune response to both the hapten dinitrophenol (DNP) and the carrier (HGG) were suppressed when recipients of tolerant and normal spleen cells were challenged with DNP6HGG. This indicates that T helper cells are also a target for suppression. The results presented in this paper are discussed in relation to a possible mechanism of suppression which proposes that suppressive activity represents the induction of tolerance in immunologically competent cells by HCG which is closely associated with the tolerant spleen cells.     

Functional activity of T and B lymphocytes of mice undergoing spontaneous loss of tolerance]

The functional activity of splenocytes and thymocytes of mice tolerant to sheep red blood cells was investigated one and four weeks after tolerance induction. The tolerance was achieved by cyclophosphamide. The immunocompetence of thymocytes was fully reversed in lfour week time. The functional activity of T and B lymphocytes of the spleen was also partially recovered four weeks after tolerance induction. Preliminary thymectomy weakened but did not prevent completely the immunocompetence of T cells of the spleen from being recovered. No Tsuppressants were found in the thymus or spleen of the tolerant animals.     

Property of class I H-2 alloantigen-reactive Lyt-2+ helper T cell subset. Abrogation of its proliferative and IL-2-producing capacities by intravenous injection of class I H-2-disparate allogeneic cells

The present study investigates the distinctiveness of Class I H-2 alloantigen-reactive Lyt-2+ helper/proliferative T cell subset in the aspect of tolerance induction. Primary mixed lymphocyte reactions (MLR) revealed that Lyt-2+ and L3T4+ T cell subsets from C57BL/6 (B6) mice were exclusively capable of responding to class I H-2 [B6-C-H-2bm1 (bm1)]- and class II H-2 [B6-C-H-2bm12 (bm12)]-alloantigens, respectively. Anti-bm12 MLR was not affected by i.v. injection of bm12 spleen cells into recipient B6 mice. In contrast, a single i.v. administration of bm1 spleen cells into B6 mice resulted in the abrogation of the capacity of recipient B6 spleen and lymph node cells to give anti-bm1 MLR. This suppression was bm1 alloantigen-specific, since lymphoid cells from B6 mice i.v. presensitized with bm1 cells exhibited comparable anti-bm12 primary MLR to that obtained by normal B6 lymphoid cells. Such tolerance was rapidly (24 h after the i.v. injection of bm1 cells) inducible and lasting for at shortest 3 wk. Addition of lymphoid cells from anti-bm1-tolerant B6 mice to cultures of normal B6 lymphoid cells did not suppress the proliferative responses of the latter cells, indicating that the tolerance is not due to the induction of suppressor cells but attributed to the elimination or functional impairment of anti-bm1 proliferative clones. The tolerance was also demonstrated by the failure of tolerant lymphoid cells to produce IL-2. It was, however, found that anti-bm1 CTL responses were generated by tolerant lymphoid cells which were unable to induce the anti-bm1 MLR nor to produce detectable level of IL-2. These results demonstrate that class I H-2 alloantigen-reactive Lyt-2+ Th cell subset exhibits a distinct property which is expressed by neither Lyt-2+ CTL directed to class I H-2 nor L3T4+ Th cells to class II H-2 alloantigens.     

Radiosensitivity of the helper cell function.

The helper function of T cells primed and irradiated in vivo was tested in vitro by the Mishell-Dutton technique. Spleen cells from mice carrier-primed with HRBC and exposed to 50 to 2000 rads of x-radiation were assayed for their ability to help syngeneic normal spleen cells to mount an in vitro anti-hapten antibody response after stimulation with the conjugate TNP-HRBC. The anti-TNP response was evaluated by the Jerne technique. The helper activity was titrated by adding graded numbers of carrier-primed spleen cells to a constant number of normal spleen cells. The slope of the initial linear portion of the response-cell dose titration curve was taken as an estimated of the helper activity and found to decrease with increasing the x-ray dose. The curve describing the remaining helper activity as a function of the radiation dose shows the presence of two components, one radiosensitive, the other, radioresistant. This suggests the existence either of helper cells at different stages of activation or of two cell subpopulations participating in the helper function.     

Suppressor cells in tolerance to HGG: kinetics and cross-suppression in high dose tolerance--absence in low dose tolerance.

Spleen cells from mice made tolerant with high doses of human gamma-globulin (HGG) specifically suppress the immune response of normal, syngeneic, spleen cells. These suppressor cells were found to be cross-reactive in that they would suppress the immune response of normal spleen cells to bovine gamma-globulin (BGG) as well as to HGG. In contrast, suppressor cells could not be demonstrated in spleens of mice made tolerant with low doses of HGG (i.e., T-cell tolerance), nor could they be found in high dose tolerant mice following a second injection of DHGG at a time when the initial suppressor activity had waned. The role of suppressor cells in the induction, maintenance, and loss of tolerance is discussed.     

Generation of T helper cells in vitro. IV. F1 T helper cells primed with antigen-pulsed parental macrophages are genetically restricted in their antigen-specific helper activity.

A sequential culture technique for the in vitro induction and subsequent assay of T helper cells is employed to examine the histocompatibility requirements for antigen recognition by murine T helper cells. F1 T cells are primed in vitro with antigen-pulsed parental strain macrophages and tested for antigen-specific helper activity in cultures containing anti-Thy 1.2 serum and C treated spleen cells from hapten-primed parental or F1 mice. A semiallogenieic system is used and appropriate controls are included to avoid possible complicating effects of allogeneic interactions. The results indicate that F1 T helper cells preferentially stimulate carrier-specific anti-hapten plaque-forming cell responses in spleen cells which are H-2 identical with the macrophage used initially to prime the T cells. Parental spleen cell cultures do not respond to F1 T helper cells which were primed with the other parental strain macrophage. Supplementing this culture with macrophages which are histocompatible with those used to prime the F1T cells is sufficient to restore T helper cell activity. Thus, the genetic restriction described here is between the primed T cell and the macrophage used to elicit secondary responses and not between the T cell and B cell. The results in this semiallogeneic system, however, do not rule out the possibility of additional allogeneic genetic restrictions in the subsequent interaction of T cells with B cells.