Priming by microbial antigens from the intestinal flora determines the ability of CD4+ T cells to rapidly secrete IL-4 in BALB/c mice infected with Leishmania major. 
Infection of BALB/c mice with Leishmania major results in the rapid accumulation of IL-4 transcripts within CD4(+) T cells that react to the parasite Leishmania homologue of mammalian RACK1 (LACK) Ag.  Because memory/effector cells secrete IL-4 more rapidly than naive cells, we sought to analyze the phenotype of these lymphocytes before infection.  Indeed, a fraction of LACK-specific CD4(+) T cells expressed a typical CD62 ligand(low)CD44(high)CD45RB(low) phenotype in uninfected mice.  LACK-specific T cells were primed in gut-associated lymphoid tissues by cross-reactive microbial Ags as demonstrated by their reactivity with bacterial extracts and by the ability of APCs from the mesenteric LN of BALB/c mice to induce their proliferation.  Also, mice in which the digestive tract has been decontaminated exhibited a reduced proportion of LACK-specific T cells expressing a memory/effector phenotype and did not exhibit the early accumulation of IL-4 transcripts induced by L. major.  Thus, LACK-specific T cells represent a subset of CD4(+) T cells which have acquired the ability to rapidly secrete IL-4 as the result of their priming by cross-reactive microbial Ags.  Tracking the fate of these cells may provide information about the regulation of cell-mediated immune responses to gut Ags in physiological and pathological situations.