RESIDUAL MHC CLASS-II EXPRESSION ON MATURE DENDRITIC CELLS AND ACTIVATED B-CELLS IN RFX5-DEFICIENT MICE

Patients with major histocompatibility complex class II (MHC-II) defic iency are known to carry mutations in either the RFX complex or the tr ans-activator CIITA. While the pivotal role of CIITA for MHC-II gene t ranscription is supported by the essential absence of MHC-II molecules in CIITA-deficient mice, we demonstrate here that RFX5(-/-) mice reta in expression of MHC-II in thymic medulla, mature dendritic cells, and activated B cells. Nevertheless, RFX5(-/-) mice develop a severe immu nodeficiency due to the lack of MHC-II in thymic cortex, failure of po sitive selection of CD4(+) T cells, and absence of MHC-II on resting B cells and resident or IFN gamma-activated macrophages. This different ial requirement for CIITA and RFX5 in subsets of antigen-presenting ce lls may be specific for the mouse; it may, however, also exist in huma ns without having been noticed so far.