Potent effects of low levels of MHC class II-associated invariant chain on CD4+ T cell development.

Invariant chain (Ii)-negative mice exhibit defects in MHC class II assembly and transport that results in reduced levels of surface class II, altered antigen presentation, and inefficient positive selection of CD4+ T cells. Many CD4+ T cells that do mature in Ii-negative mice express a cell surface phenotype consistent with aberrant positive selection or peripheral activation. Reconstitution of these mice with low levels of either the p31 or p41 form of Ii does not restore transport of the bulk of class II or class II surface expression, but surprisingly does restore positive selection as measured by numbers and surface phenotype of CD4+ T cells. Thus, an Ii-dependent process, independent of effects on class II surface density, appears to be required for normal positive selection of CD4+ T cells.