James E. Riggs

Porphyromonas gingivalis: keeping the pathos out of the biont

The primary goal of the human microbiome initiative has been to increase our understanding of the structure and function of our indigenous microbiota and their effects on human health and predisposition to disease. Because of its clinical importance and accessibility for in vivo study, the oral biofilm is one of the best-understood microbial communities associated with the human body. Studies have shown that there is a succession of select microbial interactions that directs the maturation of a defined community structure, generating the formation of dental plaque. Although the initiating factors that lead to disease development are not clearly defined, in many individuals there is a fundamental shift from a health-associated biofilm community to one that is pathogenic in nature and a central player in the pathogenic potential of this community is the presence of Porphyromonas gingivalis. This anaerobic bacterium is a natural member of the oral microbiome, yet it can become highly destructive (termed pathobiont) and proliferate to high cell numbers in periodontal lesions, which is attributed to its arsenal of specialized virulence factors. Hence, this organism is regarded as a primary etiologic agent of periodontal disease progression. In this review, we summarize some of the latest information regarding what is known about its role in periodontitis, including pathogenic potential as well as ecological and nutritional parameters that may shift this commensal to a virulent state. We also discuss parallels between the development of pathogenic biofilms and the human cellular communities that lead to cancer, specifically we frame our viewpoint in the context of ‘wounds that fail to heal’.

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‐γ 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.

X-chromosome-linked immune-deficient mice have B-1b cells

The B lymphocyte subsets of X‐chromosome‐linked immune‐deficient (XID) mice were examined by flow cytometric analyses of spleen and peritoneal cells. As shown in prior studies, young adult XID mice had reduced representation of the CD5+ (B‐1a) subset in their peritoneal cavity. However, the CD11b+ (B‐1b) B‐cell subset was present and exhibited the IgMhi CD45lo CD23− phenotype characteristic of most B‐1 cells. Although present at a lower frequency than that found in their normal counterparts, B‐1b cells were evident in CBA/N and (XD2J)F1 male mice. With increasing age, B‐1b cell number increased and in the oldest XID mice were present as B‐cell chronic lymphocytic leukaemia. These results show that XID mice do have B‐1 cells, particularly the B‐1b subset.

Age-related changes in mature CD4+ T cells: cell cycle analysis.

T cell proliferative responses decrease with age, but the mechanisms responsible are unknown. We examined the impact of age on memory and naive CD4(+) T cell entry and progression through the cell cycle using acridine orange to identify cell cycle stage. For both subsets, fewer stimulated cells from old donors were able to enter and progress through the first cell cycle, with an increased number of cells arrested in G(0) and fewer cells in post G(0) phases. The number of dead cells as assessed by sub-G(0) DNA was also significantly greater in the old group. CD4(+) T cells from old mice also exhibited a significant reduction in clonal history as assessed by CFSE staining. This was associated with a significant decline in cyclin D2 mRNA and protein. We propose that decreases in cyclin D2 are at least partially responsible for the proliferative decline found in aged CD4(+) T cells.

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.

Age‐dependent increase of peritoneal B‐1b B cells in SCID mice

The impact of increasing age upon immunoglobulin production and B‐lymphocyte generation in ‘leaky’ severe combined immune‐defective (SCID) mice was examined by enzyme‐linked immunosorbent assay and flow cytometry. By 1 year of age, the mice had normal numbers of B cells in their peritoneal cavity, while their spleen had very few immunoglobulin M‐positive (IgM+) cells. The majority of B cells expressed the CD11b marker characteristic of the B‐1b subset. B‐1a (CD5+) cells were present at a lower frequency and B‐2 cells were absent. The frequency of mice producing detectable immunoglobulin increased with age, and isotype diversity within individual mice was variable. IgM production was most frequently observed followed by IgG3 and IgG2a, then IgG1, and finally IgA. The selective persistence of the B‐1 B‐cell subset in the peritoneal cavity of aging SCID mice is a natural model for the study of those genetic and environmental influences that determine lymphocyte longevity.

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.

Recipient age determines the success of intraperitoneal transplantation of peritoneal cavity B cells

In vivo studies of lymphocyte biology have used intravenous (i.v.) injection as the primary mode of cell transfer, a protocol consistent with the anatomic distribution of most lymphocytes. However, for study of peritoneal cavity B cells, i.v. injection does not correlate with anatomical localization. This report describes the restoration of B‐cell function in B lymphocyte‐defective X‐chromosome‐linked immune‐defective (XID) mice after intraperitoneal transfer of immunoglobulin heavy chain (Igh)‐disparate peritoneal cavity (PerC) cells. In contrast to i.v. transfer, intraperitoneal (i.p.) transfer restored B‐cell function in young, but not adult (>8 weeks), XID mice. When host and donor Igh allotype matched, PerC B‐cell engraftment was noted in older recipients; this reconstitution however, was also age‐dependent. Migration from the peritoneum to systemic circulation was necessary for serum IgM production as shown by the presence of donor antibody‐secreting cells in the host spleen. Host lymphocytes also influenced the success of i.p. transplantation as severe combined immune‐deficient mice, regardless of age, exhibited donor serum IgM production. Recipient age, Igh allotype, and immune‐deficiency were found to have an impact on the ability of i.p.‐transferred PerC B cells to restore B‐cell function in XID mice.

CD28 ligation increases macrophage suppression of T-cell proliferation.

When compared to spleen or lymph node cells, resident peritoneal cavity cells respond poorly to T-cell activation in vitro. The greater proportional representation of macrophages in this cell source has been shown to actively suppress the T-cell response. Peritoneal macrophages exhibit an immature phenotype (MHC class IIlo, B7lo) that reduces their efficacy as antigen-presenting cells. Furthermore, these cells readily express inducible nitric oxide synthase (iNOS), an enzyme that promotes T-cell tolerance by catabolism of the limiting amino acid arginine. Here, we investigate the ability of exogenous T-cell costimulation to recover the peritoneal T-cell response. We show that CD28 ligation failed to recover the peritoneal T-cell response and actually suppressed responses that had been recovered by inhibiting iNOS. As indicated by cytokine ELISpot and neutralizing monoclonal antibody (mAb) treatment, this ‘cosuppression’ response was due to CD28 ligation increasing the number of interferon (IFN)-γ-secreting cells. Our results illustrate that cellular composition and cytokine milieu influence T-cell costimulation biology.Cellular & Molecular Immunology advance online publication, 23 April 2012; doi:10.1038/cmi.2012.13

B-1 B cell subset composition of DBA/2J mice.

Studies of B cell subpopulations have focused upon BALB/c mice and related strains. The B cell subset composition of DBA/2J mice, a prototype strain for BALB/c mice, has been investigated less thoroughly. This report provides the results of a study of the B-1 B cells of DBA/2J mice. In contrast to C.B-17 mice, in which B-1 B cells expressed both the CD5 and CD11b antigens, CD11b expression was most characteristic of DBA/2J B-1 B cells. This was particularly evident in the peritoneal cavity where CD5-CD11b+ B cells were the predominant B cell subpopulation. The number of B-1 B cells increased with age in both the spleen and peritoneal cavity. Strain-specific differences in B cell subset composition may be significant when considering B cell lymphomagenesis with aging.