Porcine alveolar macrophages: poor accessory or effective suppressor cells for T-lymphocytes

Porcine Alv-Mø from bronchoalveolar lavages were tested for their function in an in vitro foot-and-mouth disease virus (FMDV)-specific lymphoproliferative recall response. The Alv-Mø were seen to be poor accessory cells when compared with peripheral blood monocytes. This poor capacity was evident despite an efficient expression of SLA-DR region antigens, and other co-stimulatory adhesion molecules. It was noted that Alv-Mø secrete relatively little interleukin 1 (IL-1β), with or without LPS induction, even though mRNA for the cytokine could be detected. In contrast, blood monocytes with their effective accessory activity were potent secretors of IL-1. Although this IL-1β would be important with respect to the accessory capacity of monocytic cells, it was noted that the absence of bioactive IL-1 from the Alv-Mø cultures was not solely responsible for their poor accessory function. In fact, the Alv-Mø produced factors which not only inhibited IL-1 bioactivity, but were also responsible for a clear suppression of lymphoproliferation. This suppressor activity was dependent on the type of monocytic cell present in the culture, being more prominent when “scavenger” phagocytes were present. Thus, the major role of Alv-Mø is not as an accessory cell akin to monocytes, but as both a scavenger cell, related to Mø derived from monocytes in the absence of inflammatory signals, and an immunoregulatory cell.