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Characterizing a soluble survival signal for activated lymphocytes from CD14+ cells

T‐cell activation requires at least two signals: antigen and a costimulatory signal. As antigen‐presenting cells play an important role in this area, the role of CD14+ cells in T‐cell activation, proliferation and activation‐induced cell death (AICD) was investigated. Using phytohaemagglutinin (PHA) activation, it was found that CD14+ cell depletion resulted in significantly greater AICD, decreased lymphocyte growth and up‐regulated interleukin‐2 (IL‐2) secretion. However, T‐cell activation was delayed according to the expression of CD69 and CD25. Dynabeads conjugated with anti‐CD14 monoclonal antibody (mAb) bound CD14+ cells and induced secretion of IL‐1β, tumour necrosis factor‐α (TNF‐α), transforming growth factor‐β (TGF‐β) and IL‐6, but not IL‐2, IL‐12 or IL‐15. Supernatants were collected from Dynabeads‐activated CD14+ cell cultures and designated as ‘CD14 cocktails’. Addition of CD14 cocktails to CD14+ cell‐depleted mononuclear cell cultures reversed the increased AICD, decreased lymphocyte growth and increased IL‐2 secretion. Depletion of IL‐1β and TNF‐α in the CD14 cocktails by panning followed by blocking with the corresponding mAbs had no effect on the active AICD protection. TGF‐β was determined not to be the active factor owing to the presence of >1·0 ng of TGF‐β in the media for culturing both CD14+ and CD14− peripheral blood mononuclear cells (PBMC). The CD14 cocktails did not contain IL‐12 and IL‐15. Depletion of IL‐6 with panning followed by blocking residual IL‐6 with anti‐IL‐6 mAb significantly reduced the protective effect of the CD14 cocktails. Human recombinant IL‐6 also partially reversed the effects of CD14+ cell depletion on AICD, lymphocyte growth and IL‐2 secretion. The data suggest that IL‐6 is one of the active factors in the survival signal from CD14+ cells.