P197 Cumulative weak HLA antibodies. relevant or not. A case study

Abstract

Aim The relevance of cumulative, low MFI anti-HLA specificities on equivocal virtual crossmatch (VXM) results has been discussed in the Guidelines for the detection and characterization of clinically relevant antibodies in allotransplantation of the British Society for Histocompatibility and Immunogenetics. Moreover, some labs take this phenomenon into consideration in their daily practice. However, no conclusive evidence has been published and no consensus has been reached about it. This case describes a 3 year old male awaiting cardiac transplant presenting with multiple HLA Class I and II antibodies (Ab). It demonstrates the relevance of weak HLA Ab cumulative effect on B lymphocyte flow crossmatch (FC XM) outcome. Methods Ab screening was performed according to manufacturer instructions using LabScreen Single Antigen Beads (SAB) (One Lambda). FC XM were acquired on BC FC500 and analyzed using MESF shift. MESF shift scoring validated in our lab are 100000 (strong positive). Results To assess potential for reactivity to lower MFI specificities, a mock crossmatch was performed against an individual for which the patient had Ab against 4 and 7 HLA Class I and Class II antigens, respectively. MFI for these specificities ranged between 1533 to 2908 MFI which are considered as weak positive according to the cut off stabilished in our lab for SAB test; >1500 (negative), 1500–3500 MFI (weak positive), 3500–8000 (positive), >8000 (strong positive). Cumulative MFI for Class I and Class II were 7935 and 15689 respectively. FC XM for T cells was negative (1309 ΔMESF) while it was positive for B cells (23098 ΔMESF). Conclusion In our experience, samples exhibiting specificities with MFI ranging from 1500 to 9000 detected in the SAB test remain in an uncertain zone when predicting FC XM results. The cumulative effect of specificities with weak positive MFI is not completely understood yet. This case study illustrates the contribution that this effect may have on positive B cell FC XM results, highlighting the potential for deleterious effect of multiple lower MFI HLA Ab in organ transplantation as well as the relevance of this phenomenon for VXM prediction.