2 COMPLEMENTATION GROUPS ACCOUNT FOR MOST CASES OF INHERITED MHC CLASS-II DEFICIENCY

MHC class II immune-deficiency is a rare autosomal recessive disease d ue to a defect in transacting genes, which control the expression of t he entire family of MHC alpha and beta class II genes. Previous analys es classified cells from eight MHC class Il-deficient patients and fou r experimental mutant cell lines into four complementation groups, poi nting to the existence of a large number of regulatory genes. We condu cted fusion experiments with cell lines from two-thirds of all known p atients and found that two complementation groups accounted for 20 of the 22 cases studied. These two complementation groups correspond clos ely to two ethnic groups: most patients of north African origin were c lassified into one group, while all patients originating from Spain we re classified into a second main group. This suggests the existence of restricted number of ancestor mutations leading to this disease.