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Abstract

A unique class of T lymphocytes has been identified in the spleen of normal mice. This splenic T cell population, which expresses the αβ TCR associated with the CD3 complex yet lacks the CD4 and CD8 coreceptor molecules, constitutes approximately 0.1-0.3% of total nucleated cells and 17- 22% of CD4[sup]- CD8[sup]- HSA[sup]- Mac-1[sup]- cells in the normal murine spleen. Phenotypic analysis of splenic TCR αβ[sup]+ CD4[sup]- CD8[sup]- HSA[sup]- Mac-1[sup]- (DNαβT) cells demonstrated the expression of Lyt-1, Pgp-1, ICAM-1 and the transferrin receptor whereas minimal to no B220 or YE1/19 expression was observed. Amongst splenic DNαβT cells there is a higher frequency of V[sub]β8 usage in comparison with mature SP T cells. Similar to TCR αβ[sup]+ DN thymocytes, splenic DNαβT cells proliferate in the presence of IL-7 and this response is enhanced by the addition of IL-1. CD4[sup]+ and CD8[sup]+ SP T cells, in contrast do not respond to IL-7 alone or in combination with IL-1. Splenic DNαβT cells are considered to be mature based on the expression of the heat stable antigen. Furthermore, this splenic T cell subpopulation expresses a functional T cell receptor as suggested by their responsiveness to crosslinking of the TCR associated CD3 complex which is comparable to that of mature SP T cells. In contrast, splenic TCR γδ[sup]+ DN T cells were nonresponsive to anti-CD3 crosslinking despite the expression of the TCR/CD3 complex. The addition of IL-1, however, appeared to restore the responsiveness of TCR γδ[sup]+ DN towards the activation of the TCR/CD3 complex. The functional maturity of the splenic DNαβT cell population was confirmed by their ability to express cytokine specific mRNA. DNαβT cells constitutively express IFN[sub]γ message whereas IL-4 mRNA was induced by crosslinking of the TCR associated CD3 complex. SP T cells, in contrast, express messages for IFN[sub]γ and IL-4 as well as IL-2 only in response to anti-CD3 crosslinking. The above results support the hypothesis that splenic DNαβT cells represent a unique class of mature T cells which are related to the phenotypically similar cells found in the thymus yet are distinct from the classical type of mature T cell which expresses the SP phenotype.