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Function of surface-adherent cells in the induction of the human mixed leucocyte reaction. I. The transient surface adherence of responding lymphocytes in the human mixed leucocyte response.

The aim of this study was to establish the temporal requirement for adherent cells in the mixed leucocyte reaction (MLR). At various intervals after co-cultivation, mixed leucocytes from two unrelated donors were separated into adherent and nonadherent cells and the MLR response of each population assayed on day 6. After 1 hr of co-cultivation adherent cells had a high MLR response, whereas the nonadherent cells were unresponsive and could not be restimulated by the addition of allogeneic leucocytes or syngeneic adherent cells. In contrast, the MLR was positive when syngeneic adherent cells were added to nonadherent cells which had not been co-cultured prior to the initiation of culture. The MLR of adherent cells declined progressively as a function of time of co-cultivation prior to separation. Conversely, there was a progressive increase in the MLR of nonadherent cells obtained after 12, 18 and 24 hr of co-cultivation. Morphological studies revealed a higher number of lymphocytes in the early adherent cell population than at later times. We conclude that during the first few hours of co-cultivation lymphocytes with the potential of responding to allogeneic stimulation are retained in the adherent cell population. These lymphocytes are released from the adherent cells and can generate a positive MLR without further support from adherent cells. The transient adherence of responding lymphocytes after short exposure to allogeneic cells could provide a mean of separating a subpopulation of lymphocytes capable of reacting to allogeneic stimuli.