C4d-negative antibody-mediated rejection: A pathologist's perspective and clinical outcome.

Banff'13 update included C4d-antibody-mediated rejection (ABMR) as a separate entity responsible for graft dysfunction with limited clinical/prognostic implications. We present a retrospective study to determine the incidence and outcome of C4d-negative ABMR. A total of 987 renal allograft (RA) biopsies obtained from 987 RA recipients were studied from January 2013 to January 2016. All samples were subjected to light microscopy using standard stains and C4d immunohistochemistry on paraffin sections and reported according to modified Banff's criteria. Adequate biopsies with immunological injuries were categorized as Group 1: pure ABMR, Group 2: combined ABMR with concurrent T-cell-mediated rejection (TCR), and Group 3: pure TCR. Groups 1 and 2 were further subgrouped as C4d positive (Group 1a and 2a) or C4d negative (Group 1b and 2b). Graft function was measured by serum creatinine (SCr) level (mg/dL). Of the 987 biopsies, 43.3% (404) biopsies revealed immunological injury. Of these, 27.7% of the biopsies revealed pure ABMR (Group 1), 60.6% revealed combined ABMR with TCR (Group 2), and 11.3% revealed pure TCR (Group 3). The overall incidence of ABMR (pure ABMR + ABMR with TCR) was 36.27%, of which C4d-negative rejections were 18.48% and 18.7% in Group 1 and Group 2, respectively. The mean SCr at the end of three years follow-up in patients with C4d-negative rejections was comparatively higher. C4d-negative ABMR, recently included in Banff'13, has a low incidence, usually presents early after transplantation but carries better outcome than C4d-positive ABMR. However, further long-term studies are still required for knowing the clinical course over years.