Proliferative responses to a nonspecific factor produced by irradiated stimulating cells can simulate antigen-specific secondary responses in the primed lymphocyte test.

The primed lymphocyte test (PLT) was utilized to investigate the biologic relationship between alloantigens and soluble environmental antigens. Human lymphocytes primed in vitro to the soluble antigens Candida (CAN), Tetanus (TET), or Purified Protein Derivative of Tuberculin (PPD) gave a strong proliferative response when restimulated with the initial soluble antigen in the presence of irradiated peripheral blood lymphocytes (PBL) acting as antigen-presenting cells (APC). Surprisingly, the soluble antigen-primed cells as well as alloantigen-primed cells responded to other soluble antigens in the presence of APC. By testing primed cells with supernatants derived from irradiated lymphocytes plus soluble antigen, it was apparent that the responses observed were in part due to stimulation by a soluble factor produced by the irradiated "APC" in response to the soluble antigen rather than to recognition of widespread cross-reactivity by the primed cells. This "nonspecific" factor production could be diminished by increasing the dose of irradiation to the APC or by using a T lymphocyte depleted- (SRBC-E-) APC population. In addition, certain antigen-reactive T cell clones did not respond to the nonspecific factor to the same degree as the primed bulk cultures. Nevertheless, the recognition of nonspecific stimulation induced by factors produced by irradiated lymphocytes is critical in the interpretation of primed lymphocyte responses to alloantigens or soluble antigens.