Positive virtual crossmatch with negative flow crossmatch results in two cases

Area of Science:

• Transplantation immunology
• Immunogenetics
• Diagnostic assays

Background:

• Pre-transplant human leukocyte antigen (HLA) antibodies, particularly donor-specific antibodies (DSA), are linked to post-transplant rejection.
• Prospective crossmatch (XM) is crucial for DSA identification, but logistical constraints necessitate alternative methods like virtual crossmatch (VXM) using solid-phase assays (SPA).

Observation:

• Two cases (heart-lung and kidney transplant) presented with a negative flow cytometric crossmatch (FXM) but a positive VXM for presumptive DSA.
• Investigations revealed antibodies targeting a denatured epitope on HLA-B*44 antigen present on single antigen beads (SAB) used in SPA, not the native antigen on donor lymphocytes.

Findings:

• The denaturation of HLA-B*44 epitopes during SAB manufacturing appears to cause false positive VXM results.
• In one case, the antibody's clinical relevance was unclear; in the second, the recipient's antibody did not bind to native HLA-B*44:02 on donor platelets, confirming a false positive.
• Control sera reacted with native HLA-B*44:02, further supporting the false positive nature of the recipient's antibody detection.

Implications:

• Misidentification of these antibodies can lead to unnecessary treatments or inappropriate denial of organ transplants.
• Refined diagnostic strategies are needed to differentiate true DSA from antibodies against denatured epitopes on assay beads.
• Accurate antibody detection is critical for successful transplantation outcomes and patient management.