DIMINISHED EXPRESSION OF CD40 LIGAND MAY CONTRIBUTE TO THE DEFECTIVE HUMORAL IMMUNITY IN PATIENTS WITH MHC CLASS-II DEFICIENCY

Major histocompatibility complex (MHC) class II deficiency (bare lymph ocyte syndrome, BLS) is a rare primary immunodeficiency classified as a subgroup of severe combined immunodeficiency. We studied T and B lym phocyte function by examining the CD40 ligand/CD40 system in three BLS patients from two unrelated families. CD40 ligand expression by maxim ally activated BLS T cells was diminished. This abnormality may repres ent immunological naivete rather than a general T cell defect, since e xpression of activation marker CD69 and proliferative responses to PHA or anti-CD3 were normal, and BLS T cells primed and restimulated in v itro expressed normal amounts of CD40 ligand. BLS B cells proliferated and produced IgE if stimulated with anti-CD40 or soluble CD40 ligand and IL-4. Activation of BLS B cells with soluble CD40 ligand and IL-4 induced normal expression of activation markers, although MHC class II expression remained absent. Depressed antibody titers, lack of amplif ication and failure to undergo isotype switching in response to immuni zation with bacteriophage Phi x 174 demonstrated defective T cell help . We conclude that BLS B cells are functionally normal if appropriatel y stimulated, and that the defective humoral immunity observed may be related to diminished expression of CD40 ligand on BLS T cells.