Cell-mediated immune functions in a patient with MHC class II deficiency.

This report focuses on cell-mediated immune functions in a patient with MHC class II deficiency. The patient described presented with a case of "classical" MHC class II deficiency (T and B cells within the normal range, normal lymphocyte proliferation in response to stimulation with mitogens, gene encoding for MHC class II present, no expression of MHC class II). The absence of MHC class II expression resulted in an incapability of the patient's antigen-presenting cells to function as accessory cells in the presentation of soluble protein antigens, while accessory functions required for the induction of alloantigen-induced lymphocyte proliferation or for the generation of cytotoxic T cells in response to an allostimulus were normal. The patient's T cells responded normally to alloantigenic stimulation and also had the capacity to develop antigen-specific cytotoxic functions. However, the T cells were completely naive with respect to activation by soluble protein antigens, even after presentation by accessory cells derived from the patient's healthy histoidentical brother. In this context it was interesting to note that the patient's CD4-positive cells showed a normal pattern of expression of the 4B4 marker, a marker generally present on memory T cells. These data make it tempting to speculate that in the absence of MHC class II, other cell surface structures may at least partially take over immune functions normally under the control of the MHC class II complex.