Constantin N. Baxevanis

Ia Antigens as Restriction Molecules in Ir‐Gene Controlled T‐Cell Proliferation

Restriction molecules involved in proliferative T-cell response were determined by blocking of the responses with monoclonal antibodies specific for either A (AαAβ) or E (EαEβ) molecules. Non-Ir-gene controlled responses to a large number of antigens were found to be channelled through both A and E molecules. In contrast, responses under Ir-gene control were always restricted to one of the two class II molecules (either A or E), and the class II context of recognition of a given antigen remained the same in all (or almost all) responder haplotypes. This remarkable consistency in channelling of the response to a given antigen via either A or E molecules is termed selective restriction. Selective restriction seems to operate also in the generation of suppressor T cells: thus far, we have found three Ir-gene controlled responses, in which recognition of the antigen together with E (but not A) molecules leads to Ts-cell generation.

Genetic control of T-cell proliferative responses to poly(glu40ala60) and poly(glu51lys34tyr15): subregion-specific inhibition of the responses with monoclonal Ia antibodies.

The relationship between Ir genes and Ia antigens was studied in the T-cell proliferative responses to two synthetic polypeptides poly(glu40ala60) (GA) and poly(glu51lys34tyr15) (GLT15). The response to GA was found to be controlled by an Ir gene in the I-A subregion, whereas the anti-GLT15 response was shown to be under dual control, one Ir gene mapping probably in the I-A subregion, and the other in the I-E subregion. We obtained two different lines of evidence suggesting identity of Ir and Ia genes. First, the presence of certain serologically identified allelic forms of the I-A-encoded A molecule correlated with the responder status to GA both in inbred strains and in B10.W lines, the latter carrying wild-derived H-2 haplotypes. Thus the Ir and Ia phenotypes were not separable in strains of independent origin. Second, the anti-GA response was completely inhibited by monoclonal antibodies against determinants on the A molecule (Ia.8, 15, and 19), but not by a monoclonal antibody against a determinant on the E molecule (Ia.7). In contrast, the anti-GLT15 response was only inhibited by a monoclonal antibody against the E molecule, but not by antibodies against the A molecule. Our data support the hypothesis that Ia antigens, as restriction elements for T-cell recognition, may in fact be the phenotypic manifestation of Ir genes.

Role of the Ek Molecule in the Generation of Suppressor T Cells in Response to LDHB

The role of the Ek (EαEβk) molecule in the generation of suppressor T (Ts) cells specific for lactate dehydrogenase B (LDHB) was studied using different approaches. First, lymph node cells from LDHB‐primed B1O.A(2R) (AkEk) nonresponder mice were shown to suppress the LDHB‐specific and Ak‐restricted proliferative response of T cells from the congenic responder strain B10.A(4R), which does not express E molecules (AkEo). Similarly, lymph node cells from primed CBA (AkEk) mice suppressed the anti‐LDHB response of Lyt‐1+Lyt‐2‐ T celfs (depleted of Lyt‐2‐bearing TS cells) from the same mice. Second, in vitro priming of 2R (AkEk) T cells with LDHB‐pulsed 4R (AkEo) antigen‐presenting cells (APC) generated T‐cell proliferation but not suppression. Third, nonresponder 2R mice were turned into responders by injecting them with LDHB‐pulsed 4R APC or monoclonal Ia.m7 antibody that blocks the Ek molecule. The data demonstrate that expression of Ek molecules by the APC is necessary to generate LDHB‐specific Ts cells, which in turn prevent the proliferation of Lyt‐l+Lyt‐2‐ (probably helper) cells recognizing the same antigen in the context of the Ak molecule.

Selection of H-2 molecules for the context of antigen recognition by T lymphocytes

T lymphocytes recognize the synthetic polypeptides GA and GLT and the natural antigen LDHB and are thereby stimulated to proliferate in vitro. Simultaneously with the antigen, T cells recognize class II MHC molecules of the antigen-presenting cell and the T-cell proliferation can therefore be inhibited by the addition of monoclonal antibodies specific for either A (AαAβ) or E (EαFβ) molecules. Antibody blocking of in vitro responses thus provides an opportunity to test the rules governing the selection of class II molecules (A versus E) in the recognition of different antigens. To determine these rules we tested T cells for some 40 strains (classical inbred strains and B10.W lines) carrying H-2 haplotypes derived from wild mice) for their proliferative response to GA, GLT, and LDHB. Strains that responded were then tested in the antibody-blocking assay to determine the class II context of the response. The response to GA occurred always in the context of the A molecule; no single instance was found of the response being channelled through the E molecule. Of the 19 different A molecules (A allomorphs) that could be tested, nine (47 percent) were able to provide the context for GA recognition (and hence conferred responsiveness), while the rest failed to do so (conferred nonresponsiveness). Of the 17 informative cases tested for the response to LDHB, 14 channelled the response through the A molecule, while, in the remaining cases, the cells failed to respond altogether. And again, there was no case where the response was channelled through the E molecule. However, in two instances (of 14) the E molecule provided the context for the stimulation of suppressor T cells, which then suppressed the response of helper T cells occurring in the context of the A molecule. Of the 19 cases tested for the response to GLT, eight channelled the response through the E molecule and two through the A molecule. The two cases of an E → A switch were those in which the strains failed to express cell-surface E molecules as a result of a mutation in one of the E-encoding loci. These data indicate a remarkable but puzzling consistency in the channelling of the response to a given antigen via either A or E molecules. This consistency may be a hint that there is a link between the specificity of antigen (nonself) and MHC (self) recognition by T lymphocytes.

Mechanism of H-2-Controlled Interaction Between LDHB- Specific Helper and Suppressor T Cells

Mouse strains carrying the k allele at H-2 loci A α , A β , Eβ and Eα are nonresponders to the enzyme LDHB. Nonresponsiveness is caused by Ek(E α k E β k )-restricted LDHB-specific Ts cells that inhibit the proliferation of Ak (A α k A β k )-restricted LDHB-specific Th cells. The interaction between Th and Ts cells is restricted by the A region of H-2. We present here an analysis of interactions between Th and Ts cells that recognize LDH B in the context of allogeneic Mhc molecules (allo-restricted Th and Ts cells). The results demonstrate that the interaction depends on the receptor but not the H-2 haplotype of Th cells, and on the H-2 haplotype but not the receptor of Ts cells. Thus, suppression seems to be the consequence of recognition by Th cells of LDHB together with A-region products on the surface of Ts cells.