37-OR

Abstract

HLA antibodies (Ab) activate endothelial cells (EC) and trigger monocyte (Mo) adhesion through P-selectin and FcγRs. FcγRIIa is dimorphic, at position H131R. We investigated whether FcγRIIa allotype or IgG subclass affect HLA I Ab-induced Mo adherence. Mo binding to immobilized or soluble IgG was measured by microscopy and flow cytometry, respectively. The variable regions of mouse anti-HLA I (W6/32) Ab were cloned onto human constant regions to generate chimerized HLA I hIgG1 and hIgG2. Human aortic EC were stimulated with murine or chimeric HLA I Ab and the number of adherent Mo was counted. H131 homozygous Mono Mac 6 (MM6) 1) did not bind mIgG1, 2) had enhanced capacity to bind hIgG2, and 3) bound hIgG1 through FcγRI and FcγRIIa. FcγRIIa-R131 homozygous U937 1) had enhanced capacity to bind mIgG1, 2) had reduced capacity to bind hIgG1 and mIgG2a, 3) did not bind hIgG2, and 4) bound hIgG1 through only FcγRI [Table 1]. HLA I mIgG1 and mIgG2a-activated EC recruited a comparable number of U937. FcγR inhibition significantly reduced adhesion of U937 to HLA I mIgG1 and mIgG2a-activated EC. In contrast, dramatically more MM6 were recruited in response to mIgG2a compared with mIgG1. FcγR blockade effectively inhibited adhesion of MM6 to HLA I mIgG2a, but not mIgG1, treated EC. Heterozygous THP-1 Mo displayed an intermediate phenotype. FcγR polymorphisms in DSA+ transplant patients may influence outcome. Recipients with FcγRIIa-H131 allele may experience greater infiltration of FcγR-bearing myeloid cells during AMR. DSA of the IgG2 subclass, while not complement fixing, may be clinically significant in H131 homozygous patients. Thus, strategies such as IVIG which reduce interaction of HLA I IgG with FcγRs are likely to diminish macrophage, neutrophil and NK trafficking to the graft.