Investigation and prevention of artifactual staining in flow cytometric analyses of whole blood samples from patients treated with H65-RTA, an anti-CD5 monoclonal antibody conjugated to ricin A chain.

To assess the biological effect of therapeutic monoclonal antibodies (mAbs) immunoconjugates, flow cytometric assays are often performed on patients' blood samples. We report here that, using standard protocols, staining whole blood samples with mAb-fluorochromes can result in erroneous immune cell phenotyping. Blood samples were obtained from patients treated with H65-RTA, a murine IgG1 anti-CD5 mAb conjugated to ricin A chain. While a transient decrease in CD4+ and CD8+ cells was observed during the 5 days of therapy, alterations in lymphocyte phenotype were also noted, beginning around days 15-30. The most prominent effect was an apparent loss of CD4+ cells. However, if the cells were separated from plasma prior to staining, the patients' samples demonstrated their pre-treatment lymphocytic phenotypes. Co-incubation of post-treatment (day greater than or equal to 15) patients serum with lymphocytes from normal donors also resulted in artifactual staining. The effects of the post-treatment serum could be correlated with the onset of a human immune response directed against H65-RTA. Moreover, co-incubation of normal PBMC with goat anti-mouse immunoglobulin could replicate the artifactual staining patterns. Even though the human immune response was generated against a murine IgG1 mAb, there were sufficient human antibodies crossreactive with other murine IgG isotypes to induce artifactual staining with all mAb-fluorochromes tested. To minimize artifacts, cells should be separated from plasma prior to flow cytometric analysis as well as for other immunoassays involving murine mAbs.