Coculture of CLL cells with activated autologous T cells can result in CLL cell division in addition to CD38 up-regulation. CLL cells were labeled with CFSE and incubated with activated autologous T cells for 4 to 6 days. At the end of the culture period, the samples were analyzed by flow cytometry using CD19 expression to identify CLL cells. Flow cytometer plots gated on CD19⁺ cells are shown from 3 representative patient samples. Residual CLL PBMCs cultured alone show only a single CFSE peak, indicating no cell division. In the case with initial low CD38 expression (CLL02), coculture shows CD38 up-regulation but no division. In 2 cases with initial positive CD38 expression (CLL09 and CLL03), both CD38 up-regulation and division are seen in coculture.