Interference of recombinant soluble CD4 immunoglobulin G in flow cytometric measurement of CD4+ lymphocytes

Recombinant soluble CD4 covalently linked to an immunoglobulin G heavy chain (rCD4-IgG) was evaluated clinically for the treatment of human immunodeficiency virus infection. The interference of rCD4-IgG with the measurement of peripheral blood CD4 lymphocytes by whole-blood lysis flow cytometric analysis was investigated using three commercial monoclonal antibody reagents. Addition of rCD4-IgG resulted in an artifactual decrease in measured CD4 number at rCD4-IgG levels of ≥ 1 μg/mL; the threshold for this decrease was dependent on the concentration of monoclonal antibody in the commercial preparation used for the measurement of CD4. This artifactual decrease in CD4 cell count was observed in two patients who received rCD4-IgG intravenously. The apparent decrease in CD4 count was eliminated with the use of a single phosphate-buffered saline wash step before the addition of monoclonal antibody. rCD4-IgG can bind to anti-CD4 antibody and lower the measured CD4 cell count in vitro; this interference can be eliminated by a single or a double wash step and is necessary when using the whole-blood lysis flow cytometric technique of enumerating CD4 lymphocytes in patients receiving rCD4-immunoglobulin G.