P151 Eliminating false positive flow cytometric crossmatches due to monoclonal antibody therapy

Abstract

Aim Biologics are increasingly used in autoimmune, cancer, and transplant patients. Unfortunately they can also interfere in laboratory tests. Monoclonal antibodies, such as Rituximab, are well known to cause positive flow cytometry crossmatches (FCM) independent of donor specific HLA antibody (DSA). We assessed the ability of blocking antibodies to Rituximab (RTX) and Alemtuzumab (ALM) to negate interference by these biologics in the FCM. Methods FCM was performed with serum from patients receiving RTX and/or ALM. Some sera were pretreated with anti-Rituximab ( α RTX) and/or anti-Alemtuzumab ( α ALM) blocking antibodies (Bio-Rad, Inc). HLA DSA was determined using multiplex bead arrays (LabScreen Single Antigen, Thermofisher, Inc.). FCM channel shifts (CS) were compared between treated sera and sera treated with PBS as a control. Results Blocking antibodies were titrated in serum to determine the working dilution to use in subsequent studies. Sera from patients receiving RTX and/or ALM were treated with the predetermined dilution of blocking antibody. Blocking antibody pre-treatment reduced FCM CS with surrogate donors to levels comparable to PBS controls: RTX blocking: pre-treatment CS = 23 T and 302 B; post-treatment CS = 35 T and 29 B; ALM blocking: pre-treatment CS = 262 T and 236 B; post-treatment CS = 34 T and 2 B. A highly sensitized (cPRA = 99%; DSA ;= B35, Cw4, DR17, DR52, DQ5) pediatric renal transplant candidate (RTX and ALM treated) was crossmatched with their living donor using pre-transplant and 4 day post-transplant (post-RTX and ALM treatment) sera. Post-treatment CS were increased without blocking but reduced with blocking antibody treatment. Blocked FCM CS results correlated better with reduced DSA MFI summed values in contrast to the unblocked serum (Table 1). Conclusions Antibodies to RTX and ALM effectively block interference by these biologics in FCM resulting in the ability to provide useful FCM data for patient management.