Limiting dilution analysis of human, tetanus-reactive helper T lymphocytes. A rapid method for the enumeration of helper T lymphocytes with specificity for soluble antigens.

The number of helper T lymphocytes (HTL) in human peripheral blood with specificity for the soluble protein, tetanus toxoid, was estimated by limiting dilution analysis (LDA). HTL were detected via antigen-induced interleukin-2 (IL-2) production, as measured by incorporation of tritiated thymidine by an IL-2-dependent indicator cell line, CTLL-20. Culture conditions optimizing assay sensitivity are described, and the ability to detect antigen-specific HTL using this LDA technique are demonstrated. Observed HTL frequencies in healthy human donors tested for tetanus-reactive helper T cells ranged from less than 1 HTL/268,749 peripheral blood mononuclear cells (PBMC) (undetectable) to 1 HTL/1486 PBMC. The LDA technique was also used to detect frequency shifts in human peripheral blood HTL following challenge with antigen. This assay offers distinct advantages over proliferative LDA techniques in that it is rapid (requiring only 2 days), and defines an antigen-reactive T cell subset with defined function (IL-2 secretion). Furthermore, the LDA technique can be adapted for the detection of other soluble protein antigens, such as PPD and Candida albicans. In general, this LDA technique provides a reliable, quantitative index of human HTL reactivity to any of a variety of soluble protein antigens, and has clinical as well as experimental applicability.