How masked high-titer HLA antibodies change in highly sensitized patients after Hematopoietic Stem Cell Transplantation (HSCT)

Abstract

A high incidence of pre-transplant HLA antibodies (pre-tx HLA Abs) has been reported in HSCT candidates. While the association of that incidence and tx outcome has been studied, the detailed evaluation of pre- and post-tx changes that is critical for monitoring those Abs has been lacking. So we characterized pre- and post-tx HLA Abs in two highly sensitized HSCT patients who received reduced-intensity conditioning (RIC). Serum samples were screened by Single Antigen and C1q assay. IgG was considered positive with mean fluorescent intensity (MFI) >3000, weak with MFI 600–3000, C1q considered positive with MFI >1000. Dilution effect was compared between neat and 1:10. After serum dilution, only 23% of neat positive IgG stayed positive in patient 1 (pt1), 21% in pt2; but 85% and 65% of neat weak IgG became positive – the masked Abs – in the pre-tx sera of, respectively, pt1 and 2 (Fig). Correlation between IgG and C1q MFI was high in 1:10 IgG (R2 = 0.79 for pt1, R2 = 0.82 for pt2) but not in neat IgG. Comparing C1q-fixing of pre- and post-tx sera, 13% of pt1’s pre-tx Abs stayed positive 186 days post-tx while 77% of pt2’s stayed positive 30 days post-tx. Moreover, all of pt2’s neat weak IgG that were C1q-positive stayed strong C1q-positive (MFI >20,000) while a majority of neat positive IgG that were C1q-positive could not fix C1q after HSCT. This report is first to show that highly sensitized pre-HSCT pts had two distinct IgG populations: detectable low-titer Abs with low C1q-fixing ability and masked high-titer, high C1q-fixing Abs – which were the majority of Abs that kept high C1q-fixing ability post-tx. Given today’s trend of increased haploidentical and mismatched-unrelated donors and RIC, identifying those masked pre-tx Abs with appropriate serum dilution is critical if we are to avoid missing potential DSA. Persistence of pre-tx Abs, post-tx, with C1q-fixing intact, is likely due to pts’ residual immune system surviving RIC. The clinical impact of persistent C1q-positive post-tx Abs awaits further studies.Download : Download full-size image