Analysis of feline dual lymphocyte populations observed by flow cytometry

Two discrete lymphocyte populations were observed commonly on flow cytometric analysis (FCM) of feline lymphocyte subsets. The identity of these populations as small and large lymphocytes was established by correlating data from FCM with that from peripheral blood films. Dual lymphocyte populations were more likely to be seen in feline immunodeficiency virus-positive (FIV(-)+ ve) cats but their occurrence was not affected by health status, age, gender or breed. FIV(-)+ ve cats had a significantly higher proportion of large lymphocytes than FIV-negative (FIV(-)- ve) cats. However, FIV(-)+ ve cats had significantly fewer small lymphocytes than FIV(-)- ve cats but similar numbers of large lymphocytes. Lymphocyte subset analysis revealed that small lymphocytes had a greater proportion of CD4+ cells than large lymphocytes, regardless of the FIV or health status of the cat. In FIV(-)- ve cats, small lymphocytes had a greater proportion of Pan T + lymphocytes than large lymphocytes, but the converse was seen in FIV(-)+ ve cats. The proportion of CD8 + cells was higher in small lymphocytes than large lymphocytes in well FIV(-)- ve cats but this distinction was not seen in sick FIV(-)- ve cats or FIV(-)+ ve cats of any health status. Regardless of health status, FIV(-)+ ve cats had a lower absolute count of small lymphocytes which were T cells (due to lower numbers of both CD4 + and CD8 + cells) than FIV(-)- ve cats. The numbers of small B cells were similar for both FIV(-)+ ve and FIV(-)- ve cats. However, there were no differences between FIV(-)+ ve and FIV(-)- ve cats in the absolute values of any subset of the large lymphocytes, which suggested that FIV may affect only small lymphocytes. Statistically, the inclusion or exclusion of the large lymphocyte population for routine lymphocyte subset analysis did not affect the overall results. However, because there were significant differences in subset distribution between small and large lymphocytes, analysis of both groups should be included in studies examining the role of lymphocytes in disease.