Monocyte-independent T-cell activation by polyclonal antithymocyte globulins

The in vitro mitogenic properties of polyclonal antithymocyte and antilymphocyte globulins (ATG) on peripheral blood mononuclear cells were investigated. The ATG were mitogenic in a dose-dependent manner with maximal proliferation observed at 250 or 500 micrograms/ml. ATG activated blastogenesis of CD4+, CD8+, and CD57+ (NK cells) lymphocytes. The ATG induced interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) gene expression and lymphokine secretion in cell culture supernatant and IL-2 receptor (CD25) expression. At submitogenic concentrations ATG potentialized the effect of phorbol esters on T cell proliferation, but not that of calcium ionophore. The mitogenic effect of ATG was not abrogated by monocyte depletion indicating that like CD2 monoclonal antibodies (mAbs) ATG activate T cells via a monocyte-independent pathway. CD3 and CD2 mAbs which activate T cells without binding to B surface determinants stimulated the proliferation of B cells and their differentiation into immunoglobulin (Ig)-secreting cells. In contrast, ATG induced only a transient B cell activation, but failed to support B cell differentiation into Ig-secreting cells despite the secretion of IL-2. These properties shared by ATG obtained in horses or rabbits by immunization with different cell types appear to differ from those of other T cell mitogens.