Demonstration of strong enterobacterial reactivity of CD4+CD25- T cells from conventional and germfree mice which is counter-regulated by CD4+CD25+ T cells

Unfractionated CD4(+) T cells from the gut-associated lymphoid tissue (GALT) and peripheral lymph nodes are unresponsive when exposed to enterobacterial antigens in vitro. Under similar conditions, CD4(+) T cells depleted in vivo or in vitro of CD4(+)CD25(+) T cells proliferate extensively. The CD4(+)CD25(-) T cell reactivity depends on MHC class II presentation, specific TCR stimulation, CD4 ligation, and antigen processing by antigen-presenting cells. The CD4(+)CD25(-) T cells respond to autologous and heterologous enterobacterial antigens, but not to antigens from the feces of germ-free mice. Surprisingly, CD4(+)CD25(-) T cells obtained from the GALT of germ-free mice also proliferate when exposed to enterobacterial antigens, and adding back the conventional or germ-free CD4(+)CD25(+) T cells to the enteroantigenstimulated CD4(+)CD25(-) T cells abolishes proliferation. As judged from carboxyfluorescein diacetate succinimidyl ester-labeling experiments, 4-5% of the CD4(+)CD25(-) T cells respond to enteroantigen. The data show for the first time that CD4(+)CD25(-) T cells with reactivity towards the enterobacterial flora and regulatory CD4(+)CD25(+) T cells are present in both conventional and germ-free mice. The data suggest that a significant proportion of the peripheral pool of CD4(+)CD25(-) T cells express anti-enterobacterial reactivity, which, due to the presence of regulatory CD4(+)CD25(+) T cells, is kept in a quiescent state.