Physical interaction with monocytes rescues human mature CD4+ T-cell lines from anti-CD3-induced apoptosis

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-1beta, tumour necrosis factor-alpha (TNF-alpha), transforming growth factor-beta (TGF-beta) 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-1beta and TNF-alpha in the CD14 cocktails by panning followed by blocking with the corresponding mAbs had no effect on the active AICD protection. TGF-beta was determined not to be the active factor owing to the presence of >1.0 ng of TGF-beta 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.

Monocyte rescue of human T cells from apoptosis is CD40/CD154 dependent

The induction of T-cell apoptosis is regulated in part by monocytes (CD14+ cells). Human peripheral blood monocytes inhibited the spontaneous cell death of activated T cells in vitro. The inhibition of T-cell apoptosis did not require autologous monocytes. Inhibition required direct contact with monocytes and was not due to a soluble factor. Furthermore, treatment of monocytes with actinomycin D, cycloheximide and paraformaldehyde abrogated the anti-apoptotic activity of these cells. Blocking antibody to CD40 and CD154 (CD40 ligand) decreased the ability of monocytes to aid in T-cell survival, whereas, blocking LFA-1/I-CAM-1, Fas ligand and the CD4/major histocompatibility complex class II interaction did not affect the influence of monocytes on T-cell survival. This shows that monocytes rescue of activated T cells from apoptosis is dependent upon CD40/CD154 interaction.