Antigen-specific HLA-restricted human T-cell lines

The results presented provide evidence that the HLA specificity known as MT3, BR4, or Hon7 can serve as a restriction epitope for the proliferation of certain T cells responding to mumps viral antigen. This restriction determinant was found to be HLA-linked in family studies, and to segregate centromeric to a crossover between HLA-B and DR in one family. In the population studied, the specificity was found to be associated with the DR antigens DR4, DR7, and DRw9, which are known to be associated with MT3. The ability of accessory cells to present mumps antigen in the context of this supertypic restriction determinant was blocked by a monoclonal antibody specific for MT3. Since MT3 (BR4, Hon7) has been shown to be expressed on molecules distinct from DR, our experiments suggest that such molecules are functionally important in antigen presentation to T cells.     

Complexity of the supertypic HLA-DRw53 specificity: two distinct epitopes differentially expressed on one or all of the DR beta-chains depending on the HLA-DR allotype

The supertypic HLA-DRw53 specificity is associated with three allelic class II specificities defined by alloantisera: HLA-DR4, -DR7, and DRw9. The present study demonstrates the complexity of this supertypic DR specificity by comparing two DRw53-related determinants defined by the monoclonal antibodies PL3 and 109d6. For every HLA-DR4 cell line tested, both monoclonal antibodies were found to bind to the same subpopulation of DR molecules. This PL3+, 109d6+ DR subpopulation is also found on most, but not all, DR7+ cell lines with a beta-chain pattern that is identical to the beta-chain pattern of the PL3+, 109d6+ subpopulation on DR4 cell lines. However, some DR7+ cells which carry the HLA haplotype Bw57, DR7, DRw53, DQw3 were also found which completely lack the expression of the 109d6 determinant, but continue to express the PL3 determinant and some of the DRw53 determinants recognized by alloantisera. This results from the fact that the PL3 determinant is expressed on all of the DR molecules found on DR7 cells, including the distinct subpopulation of molecules that carry the HLA-DR7 determinant recognized by the monoclonal antibody SFR16-DR7. This PL3+, SFR16-DR7+ subpopulation does not carry the 109d6 determinant, demonstrating that the PL3 and 109d6 DRw53-related determinants are distinct and can be expressed on a different number of DR molecules, depending on the allotype of the cells. Blocking studies were also performed by using these monoclonal antibodies with alloreactive HLA-DR7-specific cytotoxic T cell clones. In these studies, the T cell-defined HLA-DR7 determinants were found to be carried by the same subpopulation of DR molecules recognized by the HLA-DR7-specific monoclonal antibody and not carried by the DR molecules recognized by 109d6. The DR7+ cell lines which do not express the 109d6 determinant also fail to express another supertypic determinant recognized by the monoclonal antibody IIIE3 carried on this molecule. Furthermore, no additional allelic forms of this unique DR beta-chain were found associated with the nonpolymorphic DR alpha-chain on these cells, suggesting that this DR beta-chain gene is not expressed. These cells also behave as homozygous typing cells for the Dw11 subtype of DR7 in HLA-D typing in the mixed lymphocyte culture assay. This suggests that the lack of expression of a specific class II gene may contribute additional genetic polymorphism within the known HLA-DR allotypes.     

Molecular relationships of the human B cell alloantigens, MT2, MB3, MT4, and DR5

The human class II, HLA-linked, B cell alloantigens include the HLA-DR, MB, MT, and Te determinants. Interest in the molecular relationships of these antigens has recently intensified because of their homology to the murine Ia antigens and their possible importance in disease predisposition and transplantation. We have used alloantisera with carefully defined immunochemical as well as serologic specificity, and two immunochemical techniques, sequential immunoprecipitation with analysis by SDS-PAGE and two-dimensional gel electrophoresis, to explore the molecular relationships of the MT2, MB3, MT4, and HLA-DR5 antigenic determinants. The data presented here indicate that 1) all class II molecules that bear the DR5 antigenic determinant also bear the MT2 antigenic determinant; (2) the homozygous DR5 cell line, Swei, expresses at least two structurally distinct class II molecules, both of which bear MT2: one bears the MT2, MB3, and MT4 antigenic determinants, and the second bears the MT2, but not the MB3 or MT4 antigenic determinant; and (3) the DR5 determinant is located on at least one and possibly both of these distinct class II molecules.     

HLA class II restriction of T helper cell response to cytomegalovirus (CMV). I. Immunogenetic control of restriction

All three HLA class II families (DR, DQ, and DP) are involved in restriction of helper T cell (Th) recognition of nominal antigens including CMV. Only limited studies have been described previously to determine whether restricting determinants of DR and especially DQ are subtypic to the serologically defined DR and DQ specificities, and to what extent restricting determinants are associated with Dw specificities defined in alloresponses. In the present report, we describe a large number of CMV-specific Th clones derived from two different individuals who are seropositive for CMV. Clones were classified as being DR-, DQ-, or DP-reactive based on blocking with monoclonal antibodies. DR- and DQ-restricted clones were then examined in panel studies using antigen-presenting cells (APC) expressing the Dw subtype of the restricting DR-DQ haplotype, as well as APC expressing different Dw subtypes associated with the serologically defined specificity. Unrelated specificities were also included. Our findings show that not only for DR but for DQ as well, the primary restricting determinants appear to be subtypic to the serologically defined antigen; furthermore, subtype restriction for both DR and DQ is very closely associated with single Dw specificities. In several cases in which cross-reactivity among restricting Dw specificities was observed in association with a given DR or DQ haplotype, a molecular basis could be suggested to explain the cross-reacting determinants. A small minority of the clones appeared to be CMV specific, but was restricted by a determinant(s) that is either monomorphic or minimally polymorphic.     

Ia molecular localization of the DRw52 allodeterminant and the DRw52-like determinants defined by monoclonal antibodies

DRw52 (formerly MT2) is a human Ia alloantigen that is expressed in linkage disequilibrium with DR3, 5, w6, and w8. Although there is general agreement that the DRw52 determinant resides on biochemically defined DR molecules, conflicting evidence exists regarding whether DRw52 resides on one or both DR molecules, DQ and DR molecules, or DR and BR molecules. Six anti-DRw52 allosera and three DRw52-like monoclonal antibodies were used to identify the Ia molecules that bear the DRw52 and DRw52-like determinants from DR5 and DRw6 homozygous cells. Based on these two-dimensional gel studies, the DRw52 allodeterminant appears to reside on a subset of DR molecules from DR5 and DRw6 cells. In contrast, the determinants defined by the three anti-DRw52-like monoclonal antibodies were found to reside on one DR molecule, on the second DR molecule, or on both DR molecules, respectively. Therefore, there is considerable complexity of Ia antigenic determinants that are associated with DR3, 5, w6, and w8 at the population level.     

Serological Cross-Reactivities of Murine and Human Class II Antigen Determined by Murine Xeno Anti-Human Class II Antibody

Murine anti-human class II antibodies were shown to cross-react with polymorphic determinants of murine class II antigens. The cross-reacting antibodies were raised in B10.S(9R) mice by immunizing with human nylon wool adherent cells (Ad cells) from peripheral blood leukocytes. The B10.S(9R) anti-human Ad cell antiserum bound to the molecules consisting of two chains with molecular weights of 35K and 28K dimers which were purified with a lentil-lectin column. The B10.S (9R) anti-human class II antiserum was also revealed to contain two distinct cross-reacting antibodies with polymorphic determinants of murine class II antigens coded for by the I-A subregion of the H-2. One is specific for a determinant of class II molecules coded for by I-A^b,d,q, and the other seems to be specific for class II molecules coded for by I-A^a,k,r.     

Lack of a role of monocytes in the inhibition by monoclonal antibodies to monomorphic and polymorphic determinants of HLA class I antigens of PHA-P-induced peripheral blood mononuclear cell proliferation

This study aimed at characterizing the mechanism(s) underlying the regulatory role of distinct determinants of HLA Class I antigens in PHA-P-induced T cell proliferation and the involvement of monocytes in this phenomenon. The anti-HLA-A2,A28 monoclonal antibodies (MoAb) CR11-351, the MoAb Q6/64 to a determinant restricted to the gene products of the I antigens HLA-B locus, and the MoAb CR10-215 and W6/32 to distinct monomorphic determinants of HLA Class I antigens were found to inhibit PHA-P-induced peripheral blood mononuclear cell (PBMC) proliferation in a dose-dependent fashion. The inhibition is specific and reflects neither inhibition of PHA-P binding to cells nor a toxic effect of the anti-HLA Class I MoAb. The latter differed in the concentration required to induce inhibition, in the influence of the concentration of PHA-P used as mitogen, in the differential effect on the donors used as a source of PBMC, and/or in the requirement of the Fc portion to induce inhibition. At variance with the information in the literature, the inhibitory effect of anti-HLA Class I MoAb on PHA-P-induced PBMC proliferation neither reflected their interaction with accessory cells nor was mediated by suppressor factors released by monocytes stimulated with PHA-P in the presence of anti-HLA Class I MoAb. Therefore, the regulatory role of HLA Class I antigens in T cell proliferation is not likely to be mediated by monocytes and/or factors released from them, but may reflect an involvement of these molecules in T cell activation pathways.     

Partial characterization of T cell components related to defined VH (VT) markers

Certain antigen-binding surface molecules and factors of T cells possess serological determinants related to immunoglobulin (Ig)-heavy-chain-variable regions (VH). We obtained sufficient quantities (greater than 100 micrograms) of homogenous VH-related T-cell molecules (VTM) for biochemical studies from normal murine thymocytes and by growing large quantities of monoclonal T-cell leukemia lines expressing the determinants. A solid phase immune adsorbent prepared from the IgG fraction of rabbit anti-IgT serum was used to isolate VTM from formic acid-solubilized T cells. The VTM from murine thymocytes and T-cell lines had Mr of 65,000-68,000. The VTM from distinct cell lines differ by isoelectric focusing and resolution of tryptic peptides indicating clonal restriction. VTM lack conventional light- or heavy-chain-constant region determinants but cross-react with antisera directed against defined VHa allotypes and JH peptides. The detection of a cross-reaction with a synthetic JH peptide is consistent with recently published data identifying JH-related sequences in putative T-cell receptor genes. The amino acid compositions of the VTM were distinct from those of mammalian Ig, major histocompatibility complex (MHC) antigens, and viral glycoproteins, but significant similarities occur with Ig V regions or heavy chains of primitive vertebrates. The results indicate that the VH-bearing T-cell products are not classical Ig, but bear limited VH-cross-reactive determinants.     

The mouse primed lymphocyte typing (mPLT) test

A murine primed lymphocyte typing (mPLT) assay, based on the sequential selective isolation of specific immunocompetent, alloantigen-reactive T blast cells, has been utilized to define the H-2-associated lymphocyte-stimulating (LS) determinants. Data obtained using mPLT cells indicate that both the Ia molecules of the J region and the SD molecules of theK or D regions possess LS determinants. Isolated Ia molecules as well as isolated SD molecules induce mPLT cell proliferation irrespective of the genetic background, thus revealing that both classes of H-2 LS antigens function in an autonomous manner. Restimulation data of mPLT cells sensitized toI-region gene products indicate that the LS determinants of the Ia molecules are the Ia specificities. However, whereas subregionI-E (I-C) determines one stimulating moiety, ia.7, subregionI-A determines multiple stimulating Ia determinants associated with each allelic product. Genetic analysis, in combination with known serology, suggests that each allelic product of theK andD regions possesses a unique LS determinant. Based on specific cross-reactivities exhibited by mPLT cells sensitized against SD molecules, the recognition of the SD-associated LS determinant appears to be distinct from the recognition of SD specificities by antibody and recognition of the target moiety by cytotoxic T lymphocytes. Thus, this mPLT assay provides a positive approach to defining the H-2 LS determinants as well as a technique for isolating cells with functionally restricted, clonal responses. Furthermore, we propose here a nomenclature for the designation of mPLT-defined LS determinants.     

Differential regulatory role of monomorphic and polymorphic determinants of histocompatibility leukocyte antigen class I antigens in monoclonal antibody OKT3-induced T cell proliferation

Monoclonal antibodies (mAb) to monomorphic and polymorphic determinants on the heavy chain of histocompatibility leukocyte antigen (HLA) class I antigens inhibit mAb OKT3-induced T cell proliferation, whereas the anti-beta 2-microglobulin mAb NAMB-1 does not affect it. The inhibitory effect of anti-HLA class I mAb is specific, is not an Fc-mediated phenomenon, does not require accessory cells, and does not involve early stages of T cell activation. Distinct determinants of HLA class I antigens regulate T cell proliferation by different mechanisms, because the anti-HLA-A2, A28 mAb CR11-351, and the mAb W6/32 to a framework determinant of HLA class I antigens block interleukin 2 (IL-2) secretion and IL-2 receptor expression, whereas the mAb CR10-215 to a monomorphic determinant blocks only IL-2 receptor expression. The mAb CR10-215 and W6/32 induced a 50% of maximal inhibition of T cell proliferation, when added after 27 and 12 hr, respectively, of incubation of peripheral blood mononuclear cells with mAb OKT3. On the other hand, the mAb CR11-351 inhibited T cell proliferation even when added after 38 hr of incubation of peripheral blood mononuclear cells with mAb OKT3 and was the only one to inhibit proliferation of cycling T lymphocytes. It is suggested that HLA class I antigens regulate T cell proliferation by interacting with cell-surface molecules involved in T cell activation. The differential inhibitory activity of the anti-HLA class I monoclonal antibodies tested may reflect the different ability of the corresponding determinants to interact with activation molecules.     

Oligosaccharide-dependent and independent Qa-1 determinants

A contribution of N-linked oligosaccharides to determinants recognized by alloreactive cytotoxic T lymphocytes has not been demonstrated. Employing cloned CTL and tunicamycin, an inhibitor of protein glycosylation, we found that carbohydrate addition was required for the formation of two of six Qa-1 determinants. The other determinants were detectable on nonglycosylated Qa-1 molecules, similar to observations in most reports that allodeterminants on class I molecules are not dependent on glycosylation for serologic detection. Examination of TM-treated, Con A-activated lymphoblasts revealed a direct correlation between the determinants defined by the reactivity of CTL clones with target cells from four Qa-1 genotypes and their dependence on carbohydrate side chains for expression. Most anti-Qa-1b CTL clones recognized either a glycosylation-dependent determinant found only on Qa-1b cells or glycosylation-independent determinants on both Qa-1b and Qa-1c cells. Similarly, clones that killed only Qa-1a cells recognized a glycosylation-independent determinant. However, clones reactive with both Qa-1a and Qa-1d cells recognized a glycosylation-dependent determinant on Qa-1a molecules and a glycosylation-independent determinant on Qa-1d molecules. This result indicates that such clones recognize cross-reactive conformational determinants, not carbohydrate itself. Thus, N-linked oligosaccharides serve to stabilize the conformation of some Qa-1 determinants, but others remain intact on nonglycosylated molecules. The absence of similar data for H-2K/D/L molecules suggest that a reexamination of other class I antigens with cloned CTL is in order to determine whether Qa-1 molecules are unique.     

Molecular localization of allogeneic and self determinants recognized by bulk and clonal populations of cytotoxic T cells

The localization of MHC-encoded determinants recognized by hapten- and allo-specific cytotoxic T cells was analyzed with the use of cell lines expressing recombinant H-2Dd and H-2Ld MHC products. Bulk cultures of CTL against TNP-self, FITC-self, and AED-self recognized self determinants associated with the N/C1 domains of both Dd and Ld products. A number of allo- and hapten-specific CTL clones were also tested for recognition of the recombinant MHC products. The allo clones specific for Ld or Dd antigens recognized the respective N/C1-associated determinants. In addition, all clones generated against H-2q and known to cross-react with H-2Dd antigens recognized determinants associated with the N/C1-associated Dd determinants. Thus, some of the results obtained with CTL parallel, whereas others contrast with, those findings obtained with monoclonal anti-H-2 antibodies. Similar to the observations made with the monoclonal antibodies, no determinant as defined by T cells has been found to be lost as a result of the interaction between the N/C1 and C2/M domains of the Ld and Dd proteins. Nor did our studies detect the presence of new antigens resulting from the interaction of these gene products. However, the present T cell findings continue to contrast previous results demonstrating that antibody interaction with class I products includes recognition of C2/M-associated epitopes.     

Role of accessory cells in B cell activation. IV. Ia+ accessory cells are required for the in vitro generation of thymic independent type 2 antibody responses to polysaccharide antigens

It has been previously shown that the in vitro antibody response to TNP-Ficoll requires the presence of adherent accessory cells. In order to determine if this characteristic was unique to TNP-Ficoll or a general feature of the TI-2 antibody responses, responses to the polysaccharide antigens TNP-Levan and TNP-Dextran were studied. Also, it was determined if the functionally relevant accessory cell expresses Ia determinants. Passage of spleen cells over Sephadex G-10 abrogated the response to TNP-Levan and TNP-Dextran as well as to TNP-Ficoll. Addition of adherent accessory cells to the G-10 passed spleen cells reconstituted the response to all 3 antigens. Pretreatment of the adherent accessory cells with a specific anti-Ia serum plus complement abrogated the ability of these cells to provide accessory cell function in the responses to all 3 antigens. Thus, an Ia-positive adherent accessory cell is required for the generation of TI-2 antibody responses to these polysaccharide antigens. This raises the possibility that genetic restrictions may exist between the Ia-positive accessory cell and the lymphocytes involved in the responses to TNP-Ficoll, TNP-Dextran, and TNP-Levan.     

Heterogeneity of tetracycline resistance determinants in Streptococcus

We found that naturally occurring tetracycline resistance in streptococci is encoded by more than one genetic determinant. Two of these distinct determinants were cloned, and the regions that are necessary and sufficient for expression of tetracycline resistance were defined by deletion analysis. These cloned determinants were further characterized by DNA-DNA hybridization experiments which also identified a third genetically unrelated tetracycline resistance determinant. Some of these genetic differences appear to represent mechanistic differences. The tetL determinant was associated with small nonconjugative plasmids and mediated resistance to tetracycline. The tetM determinant was most often "nonplasmid" associated and mediated resistance to minocycline as well as tetracycline. The tetN determinant was represented on a large conjugative plasmid and was genetically distinct from tetL and tetM, although phenotypically it resembled tetM.     

H-2-restricted graft-versus-host reaction: Foreign determinants and restriction elements

Nylon-wool-purified T cells from radiation chimeras cause a lethal graft-versus-host reaction (GVHR) in irradiated, bone-marrow-protected recipients only if the recipient shares a restriction element with the T-cell donor and also expresses antigens foreign to the donor. Class I molecules (H-2K and H-2D) can act as restriction elements, but restriction to class II molecules could not be demonstrated. However, class II molecules as well as H-2K and some non-H-2 determinants could serve as foreign antigens.     

Heterogeneity of human melanoma-associated antigens defined by monoclonal antibodies and conventional xenoantisera

Summary Immunochemical analysis of cultured human melanoma cell detergent extracts and spent culture medium with conventional xenoantisera and monoclonal antibodies identified four types of 94,000 (94K) dalton molecules and two types of high-molecular-weight melanoma-associated antigens by the following characteristics: (1) association with other components, (2) mobility in SDS-PAGE under reducing and nonreducing conditions, (3) antigenicity, and (4) presence in spent culture medium. Conventional xenoantisera were found to contain antibody populations to antigenically distinct structures, some of which have similar apparent molecular weights. Immunodepletion studies showed that the antigenic determinant detected by the monoclonal antibody 225.28S to a high-molecular-weight melanoma-associated antigen was expressed on a subpopulation of the antigens defined by the conventional xenoantiserum #8995. These data prove that antibodies reactive with antigens of similar molecular weight cannot be assumed to identify the same structures, and indicate that tumor-associated antigens may be heterogeneous in the expression of antigenic determinants defined by monoclonal antibodies.Visiting investigator from the Veterans Administration Hospital, Minneapolis, MinnesotaVisiting investigator from Sapporo Medical College (Japan). Abbreviations used: MAA, melanoma-associated antigen; PBS, phosphate-buffered saline; NP40, nonidet P40; MoAb, monoclonal antibody; SDS-PAGE, sodium dodecyl sulfate polyacrylamide gel electrophoresis; 2-ME, 2-mercaptoethanol     

Evidence for polymorphism of MB3 antigens among three HLA-D clusters associated with HLA-DR4

In a previous study, we showed that the three hitherto serologically indistinguishable HLA-D specificities associated with HLA-DR4, HLA-DYT, HLA-DKT2, and HLA-Dw4 can be distinguished on the basis of their reactivity with two distinct la-like-specific monoclonal antibodies, HU-18 and HU-23. In this study, we attempted to identify and characterize Ia-like molecules recognized by HU-18 and HU-23 on a molecular level because la subsets (HLA-DR, MB, MT, or SB) identified by them remained unknown. The results of sequential coprecipitation assays and two-dimensional gel analyses showed that both HU-18 and HU-23 recognize antigenic determinants borne on M133 but not on HLA-DRw6.2 molecules. Because the two monoclonal antibodies, specific for determinants carried on MB3 molecules, show distinct reactivity against homozygous typing cells defining HLA-DYT, HLA-DKT2, and HLA-Dw4, all of which share DR4-MB3, the data indicate that these three HLA-D clusters associated with HLA-DR4 possess distinct MB3 molecules, suggesting the existence of polymorphism in MB3 antigens.     

Human helper T-cell clones that recognize different influenza hemagglutinin determinants are restricted by different HLA-D region epitopes

Human T-lymphocyte clones (TLCs) were generated against the hemagglutinin (HA) of A/Texas/1/77 influenza virus by limiting dilution. TLCs were then screened for antigen specificity on chemically synthesized peptides representing the HA1 molecule. It has been hypothesized that different T cells that recognize the identical antigenic determinant are controlled by (restricted by) the same class II epitope. Two TLCs, HA1.4 and HA1.7, both recognized the same HA peptide and in proliferation studies exhibited identical restriction patterns. Two other clones, HA 1.9 and HA 2.43, recognized different HA determinants and also had distinct restriction patterns. Proliferation inhibition studies with monoclonal antibodies against human class II molecules demonstrated three unique patterns of blocking with the clones, suggesting that clones may be restricted to a unique class II epitope depending on the HA determinant recognized. These data can be interpreted as supporting the argument that human immune responses to influenza hemagglutinin are under Ir gene control exerted at the level of the viral antigenic determinant recognized in association with particular D-region restricting elements. The determinant selection and clonal deletion theories are compared for their capacity to best explain these findings.Abbreviations used in this paper ³HTdR tritiated methyl thymidine - MHC major histocompatibility complex - HLA human MHC - PBLs peripheral blood lymphocytes - APCs antigen-presenting cells - TLCs T-lymphocyte clones - TCGF T-cell growth factor - MoAbs monoclonal antibodies     

The murine Kupffer cell. II. Accessory cell function in in vitro primary antibody responses, mitogen-induced proliferation, and stimulation of mixed lymphocyte responses

The ability of murine Kupffer cells to function in several in vitro immunologic systems was investigated. These cells have been shown previously to function as accessory cells in antigen-stimulated T cell proliferation in response to protein antigens. In the present study it has been demonstrated that murine Kupffer cells also are competent as accessory cells in in vitro primary antibody responses to TNP-KLH and for T cell proliferative responses to concanavalin A. In addition, murine Kupffer cells were found to be potent stimulators of mixed lymphocyte responses. These studies extend previous observations by demonstrating that Kupffer cells are competent accessory cells in several distinct in vitro correlates of in vivo immune responses. The role of Kupffer cells in in vivo immune responses, particularly those to enterically derived antigens, may require re-evaluation in the light of these findings.     

Role of class II histocompatibility antigens in Staphylococcus aureus protein A-induced activation of human T lymphocytes

The capacity of peripheral blood monocytes and B lymphocytes to support staphylococcal protein A (SpA)-induced proliferation of autologous and allogeneic T cells, as well as the role of major histocompatibility complex (MHC) class I and II molecules in this activation process, were investigated. Highly purified peripheral T lymphocytes did not proliferate in response to SpA, but their response was reconstituted by both irradiated (or mitomycin C-treated) monocytes and B lymphocytes. The effect of B cells on the SpA-induced T-cell response could not be explained by a contamination of residual accessory cells because long-term continuous B-cell lines restored SpA-induced T-cell DNA synthesis as effectively as did monocytes. Support of SpA responsiveness by B cells could not be accounted for by polyclonal binding of SpA to cell surface immunoglobulins, since the ability of SpA-unreactive and SpA-reactive B cells was comparable. The cells from two human leukemic lines--K562 and Raji--showed the same ability in supporting the pokeweed mitogen-induced T-cell response, but the class II-positive Raji cells were much more effective than class II-negative K562 cells in restoring the T-cell responsiveness to SpA. Monoclonal antibodies specific for monomorphic determinants of MHC class II antigens, as well as their F(ab')2 fragments, consistently inhibited the SpA-induced proliferative response, whereas antibodies specific for MHC class I antigens were without effect. The antibodies specific for class II antigens appeared to act at the level of accessory cell, since pretreatment with these antibodies inhibited the ability of SpA-pulsed monocytes or Raji cells to present SpA to autologous or allogeneic T lymphocytes, respectively. These data indicate that either monocytes or normal and lymphoblastoid B cells can act as accessory cells for the proliferative response of human T cells to soluble SpA and that monomorphic determinants of MHC class II molecules play an important role in this activation process.