Optimization of antigen presentation to T cell hybridomas by purified Ia molecules in planar membranes. Ia molecule polymorphism determines the antigenic fine specificity of the response to cytochrome c peptides.

Ia molecule (Ek,b beta:Ek alpha or Ak beta:Ak alpha)-containing planar membranes were constructed with cholesterol and a 9:1 molar ratio of the phospholipids dipalmitoyl phosphatidylcholine and dilinoleoyl phosphatidylcholine. This lipid composition was found to be optimal for the stimulation of T cell hybridomas of different specificities. Use of this system allowed the detection of weak responses not measurable when other artificial membranes were used. Activation of the cytochrome c, Ek,b beta:Ek alpha-reactive hybridoma 2B4.11 using such membranes resulted in responses comparable to those found using antigen-presenting cells (APC); that is, similar amounts of IL-2 were produced at the same concentrations of antigenic peptides. Presentation of moth and pigeon cytochrome c peptides by Ek beta:Ek alpha- or Eb beta:Ek alpha-reconstituted membranes resulted in 2B4.11 response patterns similar to those previously described using B10.A or B10.A(5R) APC. These data conclusively demonstrate that differential stimulation by moth and pigeon cytochrome c peptides depends solely on structural differences in the E beta:E alpha molecules used for antigen presentation.

Quantitative analysis of the T cell response to antigen and planar membranes containing purified Ia molecules.

Planar lipid membranes containing the purified Ia molecule E beta k:E alpha k can present a peptide antigen derived from cytochrome c to the T cell hybridoma 2B4.11. The incorporation of E beta k:E alpha k into planar membranes was linear over a 120-fold range of Ia molecule concentrations, permitting the dependence of the T cell response on the Ia molecule concentration to be examined. As the Ia molecule concentration was increased in the planar membranes, two parameters changed: less antigen was needed to stimulate the T cells, and the plateau response seen at functionally saturating concentrations of antigen increased. The antigen sensitivity was analyzed by plotting the antigen concentration (log2) required to stimulate the release of 10 U of IL 2 from the T cells as a function of the Ia molecule concentration (log2). If the T cell recognized a simple unit of one antigen molecule and one Ia molecule, this plot should have generated a straight line with a slope of -1. Surprisingly, a line with a slope of -2.04 X/divided by 1.12 was observed, suggesting that the T cell might recognize one antigen molecule and two Ia molecules. This complexity, however, resulted from changes in the maximal response achieved at different Ia molecule concentrations. A similar phenomenon was observed when the Ia molecule concentration was decreased in cultures containing splenic antigen-presenting cells (APC) by the addition of an anti-E beta k:E alpha k monoclonal antibody, or the use of [B10.A(4R) X B10.PL]F1APC. The Ia molecule concentration can therefore be limiting for T cell hybridomas in cultures containing normal APC and functionally saturating amounts of antigen. When the planar membrane data were normalized to the maximal response to eliminate the effect of the changing plateau response, the resulting plot generated a line with a slope of -1.17 X/divided by 1.11. These results suggest that the sole stimulatory signal for this T cell hybridoma consisted of a 1:1 ratio of antigen and Ia molecules.