T helper subset differentiation in the absence of invariant chain

The outcome of murine infection with Leishmania major is regulated by major histocompatibility complex class II-restricted T helper cells. Invariant chain-deficient (Ii -/-) mice have impaired ability to present major histocompatibility complex class II-restricted antigens, and reduced numbers of CD4+ T cells. Despite these deficits, C57BL/6 Ii -/- mice controlled L. major infection comparably to wild-type mice. As assessed by mRNA analysis and in vitro antigen restimulation for IFN-gamma, Ii -/- mice had normal induction of Th1 subset differentiation even though antigen-dependent proliferation of their lymph node cells was substantially compromised. In addition, BALB/c Ii -/- mice exhibited a progressive course of infection and Th2 effector cell development that were comparable to that seen in wild-type BALB/c mice. We wished to determine whether this unexpected efficiency of T helper subset induction despite inefficient T cell stimulation could be modeled in vitro. In the presence of rIL-12 or rIL-4 naive parasite-specific transgenic T cells could mature into IFN-gamma-or IL-4-secreting T helper cells, respectively, even when antigen presentation was suboptimal or antigen dose was submitogenic. These experiments demonstrate that activation of T helper cells to a threshold required for IL-2 production or proliferation is not required to achieve induction of disease-regulating T helper cell effector functions, and that pathogen-associated secondary activation signals may facilitate the full differentiation of T helper subsets during limiting presentation of antigenic peptides.