Residual expression of functional MHC class II molecules in twin brothers with MHC class II deficiency is cell type specific

We examined major histocompatibility complex (MHC) class II expression in B cells, peripheral blood monocytes. activated T cells. epidermal Langerhans cells, monocyte-derived dendritic cells, dermal microvascular endothelial cells (DMEC) and fibroblasts of twin brothers with MHC class II deficiency. Although residual human leucocyte antigen (HLA)-DR expression was found on a subpopulation of epidermal Langerhans cells and a subset of peripheral b lood monocyte-derived dendritic cells, the patients B cells, monocytes and activated T cells were HLA-DR negative. After treatment with interferon-gam ma (IFN-gamma), the patients' DMEC expressed HLA-DR but not -DP and DQ at t he protein and mRNA level, whereas IFN-gamma failed to induce HLA-DR expres sion on dermal fibroblasts. The patients' monocyte-derived dendritic cells were capable of processing and presenting tetanus toxoid to autologous T ce lls. and patient-derived DMEC induced the proliferation of allogeneic CD4() T cells in an MHC class II-restricted fashion. indicating that the observ ed residual MHC class II surface expression was functional. The findings re ported show that the defect encountered in these patients is not necessaril y expressed to the same extent in different cell lineages, which is relevan t for the understanding of the patients' phenotype and also illustrates tha t only small amounts of MHC class II are needed to mount a functional cellu lar immune response in vivo.