(P204) Negative virtual crossmatch (vXM) with positive flow cytometry crossmatch (FCXM) in absence of prozone and DSA can be due to HLA-C epitope-specific antibodies.

Body: Although SAB assay has high sensitivity in detecting anti-HLA antibodies (Ab) and assigning DSA, it still has some caveats. SAB cutoffs are not standardized and vary between different labs. False detection of DAS in SAB can result in false negative vXM that can be confirmed by positive physical FCXM. Herein, we describe a case of a renal highly sensitized patient (HSP) with accumulative cPRA of 100%, that was listed for deceased donors’ (DD) for many years. After accepting two DD offers with negative vXM, both physical T- and B-cells FCXM were positive despite the absence of DSA.

Method: HLA Ab screening was performed using HLA class I/ II LABScreen Mixed Beads by Luminex. Reactivity against single antigens was further tested by LABScreen SAB. Serum dilution was used to resolve prozone effect. Epitope analysis was done using HLA Matchmaker software was used to investigate unresolved reactivates.

Results: To detriment the cause of such positive T- and B-cell FCXM, despite negative vXM in both DD offers, the prozone effect and/or Peanut Butter effect due to epitope-specific Abs against common HLA antigen in both DDs' offers were suspected. Neat and diluted and serum samples gave the same results, indicating the absence of any prozone effect. Also, HLA-C8 was found to be a common HLA antigen in both DDs' offers. Although patient has anti-C8 Abs with MFI of 2,806 which was below our HLA-C cutoff (5,000), epitope analysis was a necessity. By doing HLA-C epitope analysis, “1C” eplet was identified on C8 (Figure 1) that was shared with a patient’s pre-exciting unacceptable DSA from a previous transplant that occurred in 2011 (C7). Moreover, the patient had other “1C” eplet C-reactive Abs in the unacceptable range (C1, and C14), weak/moderate range (C12), and indeterminate range (C2, C5, C6, C8, C15, C16, and C18).

Conclusion: In this patient, HLA-C 1C-reactive eplet could not be essentially assigned as unacceptable in the initial patient's enrolment in the transplant registry program. With such reactivity against many HLA-C antigens (Table 1), the patient can only be transplanted from future self-antigen-bearing donors (i.e., C9 and/or C10) or non-1C-bearing donors (C4 and/or C17). Finally, as absence of prozone effect cannot justify negative vXM and positive physical FCXM, an epitope analysis can do.