Monocytes Suppress Immune Responses of Peripheral Blood Lymphocytes: Possible Implication of Immature Dendritic Cells

Abstract

It is impossible to raise antibody production from peripheral blood lymphocytes (PBMCs) against specific soluble antigen when immunized in vitro. However, using PBMCs pretreated with L-Leucyl-L-leucine methyl ester (LLME) before antigen stimulation, it become possible to raise antibody production effectively. In this study, we tried to identify the point of action of LLME, and further to clarify the regulation mechanisms of immune responses in the in vitro immunization. Firstly, we analyzed the population of non-treated or LLME-treated PBMCs by flowcytometry analysis, indicating that monocytes (Mo) and natural killer (NK) cells disappeared in LLME-treated PBMCs as compared with non-treated PBMCs. In these cells, only Mo could downregulate the antibody production from LLME-treated PBMCs. Mo changed its morphology and expression pattern of cell surface antigen after immunization, indicating that differentiation of Mo were induced after antigen stimulation in in vitro immunization. The expression patterns of cell surface maker antigens on the Mo in immunized PBMCs were similar to immature dendritic cell (iDQ differentiated from peripheral blood monocytes with GM-CSF and IL-4 treatment. Furthermore, both Mo in immunized PBMCs and iDC downregulated the antibody production from LLME-treated PBMCs immunized in vitro. These results indicate that Mo downregulated the antibody production of non-treated PBMCs. Moreover, these result suggeste that Mo in immunized PBMCs belongs to iDC, and Mo and iDC might play an important role in suppression of immune responses in PBMCs.