Mice lacking MHC class II molecules.

We have produced mice that lack major histocompatibility complex class II antigens, permitting us to evaluate the role of these molecules in diverse aspects of T and B cell differentiation. The mutant mice show near-complete elimination of CD4+ T lymphocytes from the spleen and lymph nodes; the few remaining CD4-positive cells are preferentially localized to B cell follicles. Surprisingly, substantial numbers of CD4 single-positive cells reside in the thymus; however, these are not mature thymocytes as we currently recognize them. B lymphocytes occur in normal numbers and are capable of terminal differentiation to plasma cells. Nevertheless, several aberrations in the B cell compartment are demonstrable: a lack of germinal centers, fewer IgM+IgD+ cells in certain individuals, reduced production of serum IgG1, and complete inability to respond to T-dependent antigens. In short, the class II-negative mice have confirmed some old ideas about lymphocyte differentiation, but have provided some surprises.