CD4 REGULATORY CELLS IN IMMUNE TOLERANCE

We have used the classic model of neonatal tolerance to investigate th e hypothesis that acquired tolerance depends on the generation of regu latory CD4 cells. Injection of neonatal BALB/c mice with semi-allogene ic CAF, (BALB/c X A/J) spleen cells induces antigen-specific tolerance (TOL) in 80% of mice. TOL mice accept fully allogeneic A/J skin graft s for >60 days. TOL mice show diminished Th1 CD4 and CD8 cell immunity against A/J in vitro. In contrast, TOL mice show increased levels of anti-A/J Th2 CD4 responses. Thus tolerance is associated with the inhi bition of Th1 CD4 and TC1 CD8 responses and the enhancement of Th2 CD4 responses. Because of this relationship, we hypothesized that regulat ory Th2 CD4 cells in TOL mice maintain tolerance by blocking activatio n of A/J-reactive TC1-CD8 cells. Using in vitro BrdU assays to measure CD8 proliferation within unfractionated cell cultures, we showed that CD8 cells from TOL mice proliferate normally to exogenous interleukin -2 (IL-2) but fail to proliferate in response to A/J cells. The additi on of exogenous IL-2 does not restore CD8 proliferation to A/J, ruling out simple CDS cell anergy. However, when CD4 cells are depleted from the cultures, IL-2 could restore the ability of A/J-reactive CD8 cell s to proliferate and to secrete IFN-gamma. Thus CD4 cells from TOL mic e inhibit IL-2 rescue of ''anergic'' A/J-reactive CD8 cells. The resul ts demonstrate a novel link between two major mechanisms of tolerance, immunoredirection and anergy.