We describe here the properties of mAb GK1.5, which recognizes a cell surface molecule designated L3T4; the determinant on L3T4 recognized by mAb GK1.5 is designated L3T4a. We present evidence here that: i) the expression of L3T4a by murine T cell clones correlates primarily with class II MHC antigen-reactivity; ii) mAb GK1.5 blocks all class II MHC antigen-specific functions (cytolysis, proliferation, release of lymphokines) by murine class II MHC antigen-reactive T cell clones, although there appears to be clonal heterogeneity in the degree to which these functions are blocked by mAb GK1.5; iii) mAb GK1.5 blocks class II MHC antigen-specific release of IL-2 from cloned T cell hybridomas by blocking class II MHC antigen-specific binding; and iv) L3T4 is very similar to the human Leu3/T4 antigen. The properties of mAb GK1.5 (complement fixation, reactivity with all mouse strains tested, profound blocking of all class II MHC antigen-specific functions by murine T cells, usefulness for FACS analyses, and usefulness for immuno-precipitation/SDS-PAGE analyses) make it suitable for investigating both the role of class II MHC antigen-reactive T cells in various immunological phenomena and the mechanistic basis, at the molecular level, of class II MHC antigen-reactivity by murine T cells.

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A panel of dendritic epidermal T cell (DETC) lines, and hybridomas derived from them, has been shown to spontaneously secrete lymphokines in the absence of added stimuli, which suggests that these cells are autoreactive. These cell lines are characterized by the expression of a V gamma 1.1C gamma 4/V delta 6 type T cell receptor (TcR), but several of the DETC lines also express a second TcR. Sequence analyses of these gamma/delta TcR revealed that the gamma chains were identical and that the delta chains, while not identical, were quite restricted in diversity, indicating that these receptors may recognize a common or closely related group of antigens. Analysis of hybridomas derived from newborn thymocytes identified six hybridomas that spontaneously secrete lymphokines. Five hybrids expressed a V gamma 1.1C gamma 4/V delta 6 receptor and one hybrid a V gamma 1.1C gamma 4/V delta 4 receptor that had a close structural relationship to the DETC gamma/delta TcR associated with spontaneous lymphokine secretion. gamma/delta TcR of the C gamma 4 type expressed by splenic hybridomas that did not spontaneously secrete lymphokines revealed no such relationship. Curiously, like the DETC, several of the thymocyte hybridomas that spontaneously secreted lymphokines expressed a second TcR, V gamma 2C gamma 1 or V gamma 3C gamma 1, apparently in association with the same delta chain that paired with the C gamma 4 chain. The presence of spontaneous lymphokine-secreting gamma/delta T cells with such highly homologous TcR in both the thymus and skin suggests a thymic origin for the autoreactive DETC and that these cells recognize a common or closely related group of self-antigens.