Transfer of cholesterol from macrophages to lymphocytes in culture

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Abstract
A major feature of macrophage metabolism is its capacity to produce and export cholesterol. Several reports have shown that the manipulation of lymphocyte cholesterol content elicits important changes in lymphocyte proliferation. These findings lead to an inquiry as to whether macrophage‐derived cholesterol released into the lymphocyte surroundings may be transferred to the latter thus affecting lymphocyte function. In this study, cholesterol transfer from macrophages to lymphocytes was examined in vitro using rat cells in culture. The findings indicate that there may be a significant transfer of cholesterol from [4‐14C]cholesterol labeled resident peritoneal macrophages to mesenteric lymph node resting lymphocytes (up to 173.9±2.7 pmol/107 lymphocytes/107 macrophages when co‐cultivated for 48 h), in a lipoprotein‐dependent manner. This represents the mass transfer of ca. 17 nmoles of cholesterol molecules per 107 lymphocytes from 107 macrophages (calculated on the basis of specific radioactivity incorporated into macrophages after the pre‐labelling period), which suggests that macrophages are capable of replacing the whole lymphocyte cholesterol pool every 21 h. Moreover, an 111%‐increase in the total cholesterol content of lymphocytes was found after co‐cultivation with macrophages for 48 h. When compared to peritoneal cells, monocytes/macrophages obtained from circulating blood leukocytes presented a much higher cholesterol transfer capacity to lymphocytes (3.06±0.10 nmol/107 lymphocytes/107 macrophages co‐cultivated for 24h). Interestingly, inflammatory macrophages dramatically reduced their cholesterol transfer ability (by up to 91%, as compared to resident macrophages). Cholesterol transfer may involve a humoral influence, since it is not only observed when cells are co‐cultivated in a single‐well chamber system (cells in direct contact), but also in a two‐compartment system (where cells can communicate but not by direct contact). Co‐cultivation with macrophages decreased the basal incorporation of [2‐14C]thymidine into lymphocyte DNA and the addition of cholesterol to lymphocyte culture media also impaired the lymphocyte proliferative response to the mitogens concanavalin A (Con A) and bacterial lipopolysaccharide (LPS). The above results suggest that macrophages may transfer cholesterol to lymphocytes (from both lymph nodes and blood), thus regulating lymphocyte function by raising the intracellular cholesterol content and suppressing lymphocyte proliferative activity. If this is so, a modulatory role for the transfer of cholesterol in both physiological (e.g. immune response) and pathological conditions (e.g. atherosclerosis) may be postulated. This hypothesis is currently under investigation in our laboratory.

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