Nonlymphoid peritoneal cells suppress the T cell response to Mls

The Peritoneal Macrophages in Inflammatory Diseases and Abdominal Cancers

Peritoneal macrophages (PMs) are the major cell type of peritoneal cells that participate in multiple aspects of innate and acquired immunity in the peritoneal cavity. PMs have an ability to release a large amount of proinflammatory and anti-inflammatory cytokines and therefore play a critical role in regulating the differentiation of innate immune cells and inflammatory T cells. Accumulating studies demonstrate that the immunological reactions and inflammatory responses of PMs are strongly related to the pathogenic processes of various inflammatory diseases and abdominal cancers. Consequently, the regulation of PM activation has gradually emerged as a promising target for immunotherapy, and better understanding of the distinctly biological function of PMs in individual diseases is crucial for designing specific and effective therapeutic agents. This review covers the characterization and immunological function of PMs in hosts with inflammatory diseases and abdominal cancers.

Peritoneal T lymphocyte regulation by macrophages

The T cell composition of the peritoneal cavity (PerC) in naïve BALB/c, C57BL/6, DBA/2J, and B-1 B cell-defective BALB.xid mice was investigated. The BALB.xid PerC T cell pool had a high CD4:CD8 T cell ratio relative to the other strains whose ratios were similar to those found in their lymph node and spleen. All mice had significant representation of T cells with an activated (CD25(+), GITR(hi), CD44(hi), CD45RB(lo), CD62L(lo)) phenotype and low numbers of Foxp3(+) T(reg) cells in their PerC. Despite a phenotype indicative of activation, peritoneal T cell responses to CD3 ligation were very low for C57BL/6 and BALB.xid, but not BALB/c, mice. Enzyme inhibition and cytokine neutralization studies revealed active suppression of the T cell response mediated by the macrophages that represent a significant portion of PerC leucocytes. Driven by IFNγ to express iNOS, macrophages suppressed T cell activation in vitro by arginine catabolism. Although BALB/c T cells were also in a macrophage-dense environment their limited IFNγ production failed to trigger suppression. This difference between BALB/c and BALB.xid PerC T cells suggests a role for xid in shaping macrophage-mediated immune regulation.

Cytokine treatment of macrophage suppression of T cell activation

High Mphi:T cell ratios suppress the immune response to the retroviral superantigen Mls by IFNgamma-triggered production of the arg- and trp-consuming enzymes iNOS and IDO. Attempts to reverse suppression by treatment with pro-inflammatory cytokines revealed that IL-6 improved the T cell response to Mls and the pro-hematopoietic cyokines IL-3 and GM-CSF increased suppression. GM-CSF treatment increased Mphi expression of CD80, a ligand for the immune suppressive B7H1 and CTLA-4 receptors. These results illustrate potential strategies for reversing the suppression of cell-mediated immunity characteristic of the high Mphi:T cell ratios found in many tumors.

Peritoneal macrophages suppress T-cell activation by amino acid catabolism

T-lymphocyte activation triggered by anti-CD3, endogenous or exogenous superantigen, and mitogens was suppressed in a cell-dose-dependent fashion by peritoneal cavity (PerC) leucocytes. Study of lymphocyte-deficient mice and the use of multiparameter fluorescence-activated cell sorter analyses revealed that macrophages were responsible for this form of immune regulation. Interferon-gamma was essential to trigger suppression, which, by enzyme inhibition studies, was shown to be the result of tryptophan and arginine catabolism. These results illustrate that macrophages, which are classically defined by their innate effector function as antigen-presenting cells, have the potential to temper adaptive immunity.