Capillary thrombosis in the skin: A pathologic hallmark of severe/chronic rejection of human vascularized composite tissue allografts?

Background. Vascularized composite tissue allografts (VCA) can undergo rejection, manifesting pathologically with skin changes that form the basis of the Banff 2007 classification of VCA rejection.Methods.We have followed 10 human VCA recipients (7 with hand allografts, 3 with face allografts) for pathological signs of rejection. All of them developed episodes of acute rejection. Two patients with hand allografts presented in some of their skin biopsies an as yet unreported pathological finding in human VCA, consisting of capillary thromboses (CT) in the upper dermis. Results. Capillary thrombosis was associated with other typical changes of grade II to III VCA rejection, namely, perivascular T cell infiltrates, but not with vascular C4d deposits (in formalin-fixed tissue). Clinically, the lesions presented as red or violaceous (lichenoid) cutaneous maculopapules. The first patient had several episodes of acute rejection during the 7-year follow-up. The second patient developed donor-specific antibodies; some months after CTwere first observed, he developed chronic rejection leading to partial amputation of the allograft. Pathological examination of the skin showed graft vasculopathy and occasional C4d deposits in cutaneous capillaries. Conclusions. Capillary thrombosis seems to be a novel pathologic finding associated with human VCA rejection. Although its mechanism (immunologic vs nonimmunologic) remains unclear, this finding could carry an unfavorable prognostic significance, prompting close monitoring of the patients for severe/chronic rejection.