T-cell expansion influences propagation of CLL cells. (A) Mean numbers of circulating T cells at 2-week intervals. CLL cells proliferate only when circulating T cells exceed a threshold (solid line); above this, additional T cells do not influence extent of CLL cell proliferation. For some cases, T-cell numbers exceed the threshold early (time point 1); for others, it takes much longer; and some the threshold is never exceeded. (B) CLL 753 as an example indicating that CLL proliferation only occurs when the threshold number is exceeded, at time point 2 (see also Figure 5D).