NUMBER OF INTERLEUKIN-4-SECRETING AND INTERFERON-GAMMA-SECRETING HUMAN T-CELLS REACTIVE WITH TETANUS TOXOID AND THE MYCOBACTERIAL ANTIGEN PPD OR PHYTOHEMAGGLUTININ - DISTINCT RESPONSE PROFILES DEPENDING ON THETYPE OF ANTIGEN USED FOR ACTIVATION

The enzyme-linked immunospot (ELISPOT) assay has been proven to be an efficient and sensitive method for the enumeration of single cells sec reting antibodies or cytokines. Here we have used this method to deter mine the number of interleukin-4 (IL-4)- and interferon-gamma (IFN-gam ma)-producing cells in in vitro secondary responses to tetanus toroid (TT) and the mycobacterial antigen (purified protein derivative; PPD) or the mitogen phytohemagglutinin (PHA). PHA-induced IL-4 and IFN-gamm a secretion was well correlated suggesting polyclonal activation of ce lls. This was not the case with the specific antigens, where PPD prefe rentially induced IFN-gamma- and very few IL-4-producing cells, while TT-induced both IL-4 and IFN-gamma. These differences are probably a r eflection of the types of immunity the two antigens induce, mycobacter ia preferentially inducing a cell-mediated T helper type 1 (Th 1) type of immunity, while immunity to tetanus is an antibody-dependent, Th 2 type of response. In individuals recently boosted with TT, a signific ant increase in both IL-4- and IFN-gamma-producing cells in response t o TT was seen at day 7 after boost, followed by decline. This was in c ontrast to what was seen in response to PPD where an increase of IFN-g amma-producing cells after the TT boost at day 7 persisted for at leas t 14 days. These results suggest that after an in vivo boost both anti gen-specific and nonspecific T cells are activated and that antigen-sp ecific cells home to other organs and therefore may be difficult to de monstrate in the circulation. Our data show that the ELISPOT assay is a powerful tool for determining the frequency of cells secreting cytok ines. The assay has several advantages over other assays since it is s ensitive, measures the number of actually secreting cells, and avoids the problems of binding of cytokines to their cell-bound or soluble re ceptors.