Ia Antigens on Antigen‐presenting Cells do not Carry the Same Ia Specificities as Detected on Suppressor and Helper T Cells

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Abstract
The passage of spleen cells through a tightly packed nylon wool column partially separated two populations of cells required for the antigen‐induced secondary proliferative response of T cells. The proliferating T cells were in the nylon wool column‐passed cell population, but the addition of a small number of nylon‐wool‐adherent cells was required for their maximal proliferative response. Such adherent cells were non‐T, non‐B, Ia antigen‐positive cells serving as antigen‐presenting cells. Ia determinants on this cell type are encoded in two separate I subregions, I‐A and I‐E/C, and they are expressed simultaneously on the same cells. Antisera specific for I‐J subregion gene products having as a known activity the killing of antigen‐specific suppressor and helper T cells were unable to kill antigen‐presenting adherent cells. The results indicated that at least I‐J subregion gene products expressed on suppressor and helper T cells are not present on antigen‐presenting accessory cells. The Ia specificities detected on accessory cells follow the cross‐reactive pattern found in Ia molecules of B cells, and thus antigen‐presenting cells carry Ia antigen identical to those of B cells, and not those of T cells.