In vitro immunosuppression mediated by an extracellular polysaccharide from Capnocytophaga ochracea. Influence of macrophages.

Abstract

The mechanism of immunosuppression produced by an extracellular polysaccharide (EP) from Capnocytophaga ochracea strain 25 (Socransky) was evaluated in vitro. Adherent (A), nonadherent (NA), and whole spleen cells (WS) from C57BL/6 inbred mice were pulsed with EP and combinations of cells evaluated for responsiveness to the T lymphocyte mitogen concanavalin A (Con A) by measurement of 3H‐thymidine uptake after 72 h of culturing. Binding of EP to spleen cells was evaluated by inhibition of 3H‐Con A binding after pre‐exposure of cells to EP. In addition, experiments were performed which attempted to block EP‐mediated suppression of responses to Con A by pre‐exposure of WS cells to 2‐mercaptoethanol (2‐ME), dithiothreitol (DTT), catalase, and indomethacin, all being known inhibitors of macrophage‐mediated immunosuppression. Significant suppression of responses to Con A occurred only when WS cells were pulsed with EP prior to exposure to the mitogen or when EP‐pulsed A cells were mixed with NA cells or WS cells prior to mitogenic stimulation. Pre‐exposure of NA cells to EP followed by mixing with untreated A cells or WS cells was comparatively ineffective in suppressing responses to Con A. EP bound to A cells to a higher degree than to NA cells as determined by inhibition of 3H‐Con A binding. Both 2‐ME and a combination of indomethacin and catalase blocked EP‐mediated immunosuppression, while preexposure of cells to DTT, catalase, or indomethacin alone was ineffective in blocking suppression. The results suggest that EP may exert its immunosuppressive activity through macrophages; however, the participation of suppressor T cells was not eliminated by these studies.