Quantification of low frequency white blood cells expressing human cytomegalovirus antigen by image cytometry

Human cytomegalovirus (HCMV) infections cause considerable morbidity and mortality in immunocompromised patients. During HCMV infection, leukocytes appear in the circulation in low frequencies that express the HCMV pp65 protein antigen. Since there is evidence that changes in the frequency of antigen-positive cells in the early phase of the infection have prognostic value, we applied automated image cytometry to quantify these antigen-positive cells. For this purpose weekly peripheral blood leukocyte samples of 80 kidney transplant recipients were visually examined for the presence of antigen-positive cells using an immunocytochemical detection method. Seventeen patients, who reacted positive with this assay, were identified. Next, automated image cytometry was applied to quantitate the frequency of antigen-positive cells in sequential blood samples from the 17 patients. Patients who developed a period of HCMV viremia had a significantly longer antigenemic period and a significantly higher frequency of antigen-positive cells than patients with a HCMV infection who remained nonviremic. Therefore, automated image immunocytometry based screening can be used to distinguish patients at risk for the development of a HCMV viremia. Moreover, automated quantitation reveals prognostic information about the HCMV infection at a more sensitive level than other HCMV detection techniques.