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Proliferation of normal (not immunized intentionally) murine spleen cells was elicited with concanavalin A, supernatant fluid from cultures of EL-4 cells, human recombinant interleukin 2 (IL-2), or a mixture of phorbol ester and calcium ionophore A23187. IL-2-induced proliferation was inhibited by membrane-permeable dibutyryl cyclic adenosine monophosphate (cAMP) or by the adenylate cyclase activator forskolin. Consistent with these observations was the finding that stimulation with IL-2 decreased and forskolin increased the intracellular content of cAMP. IL-2-induced proliferation, as well as that induced by concanavalin A or phorbol-ionophore mixture, was inhibited by monoclonal antibodies specific for L3T4 or Lyt-2 cell surface markers. This inhibition was observed even when antibodies were added several hours after exposure of cells to IL-2. Notably, antibodies did not alter the intracellular content of cAMP. Thus, the experimental data failed to establish a functional linkage between the inhibitory effect of antibodies and the regulatory effect of the adenylate cyclase system. However, our results provide a rational basis for the postulation that antibodies, upon binding to their corresponding ligands, generate a negative signal that interferes with IL-2-induced proliferation. Therefore, L3T4 and Lyt-2 molecules appear to play an important role in the regulation of lymphocyte proliferation.

Primary and secondary anti-Thy-1 responses were elicited in C57BL/6Kh and (C57BL/6Kh X BALB/cKh)F1 mice by injection of either intact or sonicated Thy-1 disparate thymocytes as the source of the two forms of Thy-1 antigen--cell bound and cell free, respectively. The primary response was characterized by a high number of anti-Thy-1 plaque-forming cells in the spleen and a high titer of serum antibodies that belonged predominantly to the IgM class. The secondary response consisted of a moderate number of plaque-forming cells and a high titer of serum antibodies that belonged to the IgG and IgM classes. Only secondary responses ensued in mice primed and boosted with the same form of antigen, whereas primary and secondary responses developed concomitantly in animals primed with one form and boosted with both forms simultaneously. These data suggested that the two forms of Thy-1 antigen activate in a given responder two discrete subsets of T cells.

The magnitude of the primary humoral anti-Thy-1.1 responses was measured by the number of PFC in spleens of B10.S(7R) mice and some of their H-2 heterozygous F1 hybrids immunized with thymocytes of either AKR.TH or B10.S(7R)-Thy-1a strain. These two of the five newly developed strains are non-H-2 incompatible and non-H-2 compatible with B10.S(7R) mice, respectively. The low anti-Thy-1.1 responses of B10.S(7R) mice and their F1 hybrids after immunization with thymocytes from non-H-2 compatible B10.S(7R)-Thy-1a donors indicated that the I-As "allele" of B10.S(7R) mice and the I-A "alleles" of other parental strains tested did not produce a complementary effect that is required for non-H-2 compatible thymocytes to elicit a good anti-Thy-1 response.

The primary immune response to the Thy-1.1 antigen was measured by a plaque assay that detected cells producing antibodies lytic for AKR thymocytes. F1 hybrids which were b/d heterozygous at the entire H-2 complex or at only some of its regions were studied. All F1 hybrids which were b/d heterozygotes at the K, IA and possibly IB regions displayed a complementary effect of the alleles at the Ir-Thy-1A and Ir-Thy-1B loci. The complementary effect was indicated by the significantly higher responses of the F1 hybrids than the responses of their parental strains. No complementary effect could be demonstrated in F1 hybrids which were b/d heterozygous at the IB, IC, S and D regions, but not at the K and IA regions. The results excluded participation of the IC, S and D regions in the phenomenon of the complementation. Furthermore, one of the Ir-Thy-1 loci could be mapped to the left of the IB subregion, most likely in the IA subregion of the H-2 complex. The second Ir-Thy-1 locus could be mapped to the left of the IC subregion in either the IA or IB subregion.