Splenic CD4+NK1.1+ T cells have been shown to secrete large and transient amounts of IL-4 mRNA 90 min after i.v. injection of anti-CD3 Ab, suggesting that this novel subset of T cells may induce Th2 responses in the spleen by quickly providing IL-4 at the onset of an immune response. beta2-microglobulin-deficient (beta2m(o/o)) mice have been shown to contain strongly reduced numbers of NK1.1+ T cells and to be severely impaired in their capacity for rapid induction of IL-4 mRNA in response to anti-CD3, demonstrating that these cells are MHC class I dependent. To address the role of CD4+NK1.1+ T cells in the induction of Th2 responses against Leishmania major, we have dissected the onset and the outcome of the immune response elicited against the parasite in BALB/c-beta2m(o/o) mice and in anti-NK1.1-treated congenic BALB/c mice expressing the NK1.1 marker (BALB/c-NK1.1+). Both BALB/c-beta2m(o/o) and NK1.1-depleted BALB/c-NK1.1+ mice developed a progressive, nonhealing disease that was indistinguishable from wild-type mice. Upon infection, early induction of IL-4 mRNA in the lymph node was not affected in BALB/c-beta2m(o/o) and in NK1.1-depleted BALB/c-NK1.1+ mice, but was abrogated by injection of a CD4-depleting Ab. These data suggest that, in the lymph node, MHC class I-dependent CD4+NK1.1+ T cells do not play a major role in the generation of Th2 responses against L. major. To investigate whether the inability of NK1.1+ T cells to induce IL-4 production in the lymph node was specific to L. major Ag, mice were challenged with low doses of anti-CD3 Ab s.c. in the footpad. In contrast to the spleen, normal levels of IL-4 mRNA were expressed in the lymph nodes of BALB/c-beta2m(o/o) mice. Thus, the MHC class I-dependent CD4+NK1.1+ cell population that gives a rapid IL-4 response in the spleen appears not to contribute significantly to early induced IL-4 responses in the popliteal lymph nodes.