Molecular specificity of a monoclonal anti-Ik alloantibody identifying a highly conserved determinant on mouse I-A, I-E and human DR antigens.

We have investigated, using serological and biochemical assays, the specificity of an A.TH anti-A.TL-derived monoclonal antibody (mAb), designated 40.B, directed at a highly conserved antigenic determinant expressed on the majority of murine and human MHC class II antigens. In the mouse, mAb 40.B defines a new specificity expressed on the Ia products of the H-2 haplotypes K, d, b, v, r, p, u, j and w3. Analysis of its reactivity with H-2 recombinant strains and the results of the competitive binding inhibition of 125I-labeled mAb 40.B to B10.BR cells with I-Ak and I-Ek specific mAb suggested recognition of a shared Ia determinant expressed on both I-Ak and I-Ek molecules. This has been confirmed by sequential immunoprecipitation studies which demonstrated the specificity of mAb 40.B for the Ak beta Ak alpha, Ab beta Ab alpha, Ad beta Ad alpha, Ek beta Ek alpha, Es beta Ek beta and Ed beta Ed alpha dimers. In humans, this mAb bound to and immunoprecipitated HLA-D/DR molecules expressed on lymphoblastoid cell lines carrying the MT1 and MT2 supertypic specificities. The possible implications of these findings with regard to an evolutionary model of MHC class II antigens are discussed.

Affinity and inhibitory capacity of T-cell proliferation of monoclonal anti-Ia antibodies.

The rates of dissociation from the surface of Ia-positive spleen cells of three different anti-Ia monoclonal antibodies were evaluated and compared with their inhibitory effects on T-lymphocyte proliferative responses involving the same spleen cell population, either as a source of accessory cells (responses to soluble antigens) or as allogeneic stimulating cells. In general, a direct relationship was observed between the affinity with which these monoclonal antibodies interact with Ia-positive cells and the magnitude of their inhibitory effects, whether soluble antigen- or alloantigen-induced responses were tested. These results indicate that the affinity of monoclonal antibodies is a factor that has to be considered to the same extent as their fine specificity when interpreting the results of inhibitory studies of T-cell responses by anti-Ia monoclonal antibodies.

Identification on I-Ak molecules of a functional site recognized by proliferating T-lymphocytes.

The inhibitory capacity of 17 monoclonal antibodies (m.Ab.) specific for the products of the I-Ak subregion was evaluated in proliferative responses of B10.BR T-lymphocytes to GAT, Keyhole limpet hemocyanin, and ovalbumin. Considered in isolation, each m.Ab. mediated inhibitory effects of comparable magnitude on these three different proliferative responses. On the other hand, clear differences were observed when the magnitude of the inhibitory effects was compared from one m.Ab. to another. The m.Ab. were consequently classified as strong or moderate-to-weak inhibitors of T-cell proliferative responses. Evidence was simultaneously gained indicating the following: (a) the determinants recognized by different m.Ab. were expressed on the same molecules; (b) the differences in affinity of the m.Ab. for I-Ak positive cells did not explain their differences in inhibitory capacities; (c) conversely, the inhibitory capacity of each m.Ab. followed its ability to inhibit the cell surface fixation of Ia.17-specific 10-2.16 m.Ab; (d) the strong inhibitory capacity of some m.Ab. was not related to a special ability to modulate cell surface Ia molecules. These results suggest that antigen recognition by T lymphocytes is preferentially restricted by a functional site of the I-Ak molecules related to the Ia.17 and Ia.1 specificities.