Hyper-IgM immunodeficiency with disseminated cryptococcosis.

We describe two siblings with X-linked hyper-IgM immunodeficiency. One patient developed disseminated cryptococcosis. Co-culture of this patient's T cells with normal B cells suppressed IgG and IgA production. The CD40 ligand gene of one patient was examined and contained a nonsense mutation at nucleotide 475. CD40 ligand is a membrane protein which is expressed on activated T cells and induces B-cell proliferation. These results suggest that there is a defect in T- and B-cell interactions in this immunodeficiency syndrome. It is also possible that patients with this syndrome are predisposed to cryptococcal infections.