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Peripheral T lymphocyte depletion by apoptosis after CD4 ligation in vivo: selective loss of CD44− and ‘activating’ memory T cells

We have demonstrated that a single intravenous bolus of rat anti‐CD4 MoAb caused a small but prolonged increase in apoptosis in murine lymph nodes. We have quantified this process using the novel Highly Optimized Microscope Environment (HOME) interactive image analysis system and shown that the increase in apoptosis was sufficient to account for the observed depiction of the peripheral CD4+ T cell subset. This occurred in the absence of any other exogenous signal. Furthermore, there was no evidence of an inflammatory or necrotic response in the tissues, indicating that this was unlikely to be He or complement‐mediated antibody killing. The anti‐CD4‐induced depiction selectively removed CD44− T cells. Using mice previously immunized with yeast‐derived HIV‐1 p24 recombinant protein there was sparing of memory T cell function after in vivo anti‐CD4 treatment, except during a window of less than 24 h duration, when simultaneous exposure to antigen and anti‐CD4 antibody resulted in the depiction of specific memory T lymphocyte function. This indicated that a very minor alteration in the frequency of apoptosis had a marked effect on cell number over time, and suggested that opportunistic infection associated with CD4+ T cell depletion may be explained by loss of memory cells when there is antigenic stimulation at the same time as CD4 ligation. These results have implications for the pathology of HIV‐associated disease which is associated with ligation of CD4 molecules in vivo.