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A rat IgM monoclonal antibody, PA3-795, inhibits the antigen-specific responses of mouse T-cell hybridomas. It recognizes a heavily glycosylated cell-surface protein, designated Lgp55, that is detectable after activation on mature T cells. During fetal life, Lgp55 is found at high levels on newly immigrant thymic T-cell precursors prior to surface expression of other T-lineage molecules. High levels of expression are also found on thymocytes in the outer cortex of adult mice. Thymocytes at later stages of differentiation bear decreasing amounts of surface Lgp55, and none is detectable on "single-positive" thymocytes in the thymic medulla or on resting mature T cells from the periphery. Addition of monoclonal anti-Lgp55 to fetal thymus organ culture decreases the output of "mature" CD4 single-positive thymocytes when it is begun before fetal day 13.5. These findings suggest that Lgp55 contributes to cell-cell interactions that regulate very early steps in T-cell development in the mouse.

We have examined the appearance in mouse ontogeny of thymocyte precursors for Ag-specific, MHC-restricted Th. These cells are first detectable at day 18 of fetal life, about 1 day after alpha/beta, TCR-positive cells begin to appear. These early Th precursors are not dependent on the thymus for priming with Ag and MHC, and are L3T4+, Lyt-2-. Thus, these cells already have the phenotype of mature Th. In neonatal F1 animals expressing both IAk and IAb, the appearance of Th precursors restricted by either IAk or IAb is specifically inhibited by treatment of the mice with anti-IAk or anti-IAb antibodies, respectively. These results indicate that cells of mature T cell phenotype and function can arise fairly rapidly from immature, receptor-bearing precursors, once these appear. Moreover the results are in line with those previously obtained in chimeric animal experiments which suggested that specific interaction of TCR on thymocytes with class II alleles in the thymus is required for the subsequent appearance of T cells restricted by those class II alleles.

KJ23a+ T cell clones, which bear the determinant encoded by the V beta 17a T cell receptor gene segment, frequently recognize IE molecules of various murine H-2 haplotypes. In the presence of IE molecules, thymic maturation of KJ23a+ clones is infrequent. We investigated the basis of this phenomenon by blocking expression of IE molecules with monoclonal anti-IE antibodies in organ cultures of fetal thymus and in neonates from the C57BR/cdJ strain (H-2k, V beta 17a homozygous). Our data support the contention that this process results from deletion of clones with anti-IE reactivity, as functional blocking of the IE molecule results in maturation of IE-reactive clones and increased numbers of KJ23a+ mature cells. In addition, we noted that blocking of functional IE expression in this haplotype permitted development of both CD4+/KJ23a+ and CD8+/KJ23a+ T cells. The CD4+ clones isolated from anti-IE-treated animals were frequently reactive against IEk; we could demonstrate no alloreactivity against B cell or B lymphoma stimulators in the CD4- clones. We conclude that clonal deletion events during thymic development may be initiated by T cell precursor interactions with MHC molecules against which the mature clones display no measurable reactivity. Specifically, clones destined to be MHC Class I-reactive may be deleted during development by interactions with MHC Class II molecules.

Fetal thymus organ culture was used to study the expression and function of antigen-specific, major histocompatibility complex-restricted receptors on thymocytes. Receptor gene rearrangement and expression occurred de novo in organ culture indicating that these events are induced in the thymus itself, presumably in response to thymus-derived stimuli. During organ culture a population of immature thymocytes expressing low levels of receptors developed first, and then diminished as mature thymocytes with high levels of receptor expression appeared. Continuous culture with antireceptor antibody modulated receptor from the surfaces of immature thymocytes, but did not prevent their appearance or accumulation. By contrast, appearance of receptor-bearing mature thymocytes was prevented in the presence of antireceptor antibody. These results indicate that the receptor is not essential for the generation of immature thymocytes but is involved in the selection or maintenance of mature cells from this pool.