Different lymphoid cell populations produce varied levels of neopterin, β2‐microglobulin and soluble IL‐2 receptor when stimulated with IL‐2, interferon‐gamma or tumour necrosis factor‐alpha
Immune activation is central to many immune disorders. Clinical investigations have shown that immune activation can be quantified by measurements of soluble immune activation products in serum. Most in vitro studies of these immune activation products have focused on single products. In this study the specific cell sources and the major lymphokines inducing multiple activation products were investigated. In vitro addition of interferon‐gamma (IFN‐γ) or IL‐2 stimulated peripheral blood mononuclear cells to produce neopterin, β2‐microglobulin (β‐M) and soluble IL‐2 receptor (sIL‐2R). These two lymphokines can act independently, because neutralizing antibodies to one of the lymphokines did not block the inducing activity of the other. Tumour necrosis factor‐alpha (TNF‐α) was also investigated and shown to be a less powerful inducer than IL‐2 or INF‐γ. Separated lymphoid subpopulations responded differently to specific lymphokines. Monocytes produced only neopterin and only in response to INF‐γ. T cells released β2‐M and sIL‐2R in response to IL‐2. B cells, however, were capable of producing all three immune activation products. Neopterin production in B cells was induced by either INF‐γ of IL‐2, indicating that B cells have additional mechanisms for responding to lymphokines. To investigate whether these in vitro findings also occur in vivo, sera from patients who had received either rIL‐2 or INF‐γ treatment were tested. INF‐γ administration led to substantial increases in serum neopterin but only a moderate β2‐M increase and no increase in the serum sIL‐2R levels. rIL‐2 administration caused a substantial increase of all three serum immune activation products, consistent with our in vitro findings. The results confirm that increased serum levels of soluble immune activation products are indicators of increased cytokine production by lymphocytes and monocytes and also that B cells can be a prominent source of immune activation products.