Qualitative and quantitative studies of antigen-presenting cell function by using I-A-expressing L cells.

I-A-expressing transfected murine L cells were analyzed as model antigen-presenting cells. Four features of accessory cell function were explored: antigen processing, interaction with accessory molecules (LFA-1, L3T4), influence of Ia density, and ability to stimulate resting, unprimed T lymphocytes. I-A+ L cells could present complex protein antigens to a variety of T cell hybridomas and clones. Paraformaldehyde fixation before but not subsequent to antigen exposure rendered I-A+ L cells unable to present intact antigen. These results are consistent with earlier studies that made use of these methods to inhibit "processing" by conventional antigen-presenting cells. The ability of anti-L3T4 antibody to inhibit T cell activation was the same for either B lymphoma or L cell antigen-presenting cells. In striking contrast, anti-LFA-1 antibody, which totally blocked B lymphoma-induced responses, had no effect on L cell antigen presentation, measured as interleukin 2 (IL 2) release by T hybridomas, proliferation, IL 2 release, or IL 2 receptor upregulation by a T cell clone. I-A+ L cell transfectants were found to have a stable level of membrane I-A and I-A mRNA, even after exposure to interferon-gamma-containing T cell supernatants. In agreement with earlier reports, a proportional relationship between the (Ia) X (Ag) product and T cell response was found for medium or bright I-A+ cells. However, dull I-A+ cells had a disproportionately low stimulatory capacity, suggesting that there may be a threshold density of Ia per antigen-presenting cell necessary for effective T cell stimulation. Finally, I-A-bearing L cells were shown to trigger low, but reproducible primary allogeneic mixed lymphocyte responses with the use of purified responder T cells, indicating that they are capable of triggering even resting T cells. These studies confirm the importance of antigen processing and I-A density in antigen-presenting cell function, but raise questions about the postulated role of the LFA-1 accessory molecule in T cell-antigen-presenting cell interaction. They also illustrate the utility of the L cell transfection model for analysis and dissection of antigen-presenting cell function.