IN THIS ISSUE

Fibrinogen-like protein 2 (FGL2) is a multipotent molecule that plays a role in innate and adaptive immunity, can mediate transplant rejection, and suppresses the action of T cells and dendritic cells (DCs). Shalev et al. (p. 249 ) used fgl2 deficient mice to show that FGL2 contributes to T regulatory cell (Treg) function. T cells from fgl2^−/− mice produced Th1 cytokines and had increased proliferation responses both to Con A and in mixed lymphocyte reactions. Additionally, B cell Ab production and DC numbers were increased in the fgl2^−/− mice. Histologically, fgl2^−/− mice had 25% fewer Peyer’s patches in the small intestine, larger spleens than littermates, and glomerulonephritis in 25% of examined kidneys. This hallmark of autoimmune complex formation coupled with the overall increased immune activity led the authors to hypothesize that FGL2 controlled Treg function. Although there were significantly more FoxP3⁺ cells in the lymphoid organs of fgl2^−/− mice, these Tregs were impaired in their ability to suppress CD4⁺ T cell proliferation. In support of a role for FGL2 in Treg activity, anti-FGL2 Ab allowed effector T cells to overcome wild-type Treg suppression and proliferate. FGL2 was also found to mediate suppression through the FcγRIIB receptor on DCs, leading to lower expression of costimulatory molecules on DCs and their eventual apoptosis. Without FGL2, Tregs cannot limit effector cell proliferation and autoimmunity results.

Estrogens have anti-inflammatory properties and the production of inflammatory cytokines, such as TNF-α, increases after menopause. Inflammation can be suppressed through both estrogen receptors (ER), but action through ERβ causes suppression without proliferation of breast or uterine tissue. Cvoro et al. (p. 630 ) have used microarray analysis to test whether estrogen acts by repressing proinflammatory cytokine gene transcription. The authors used TNF-α- and estradiol-treated human osteosarcoma U2OS cells expressing ERβ to identify which genes estrogen repressed. Of the 49 genes activated by TNF-α, 18 were suppressed by estradiol treatment and most of these were genes associated with inflammation, such as IL-6, MCP-1, and TNF-α. Synthetic ligands specific for ERβ were used and it was found that ERβ was more effective than ERα at suppressing TNF-α induced cytokine gene expression when tested in U2OS cells expressing either ERα or ERβ. The mechanism of ERβ transcriptional suppression was found to involve recruitment of the steroid receptor coactivator 2 (SRC-2). ERβ was also able to suppress LPS-induced transcription of TNF-α, MCP-1, and CSF-2 in PBMCs. With its anti-inflammatory capabilities and lack of proliferative effect, ERβ provides a potentially attractive therapeutic target for modulating inflammation.

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) typically infects otherwise healthy individuals. A dramatic increase in the incidence of infections has brought this bacterium to the attention of the popular press. CA-MRSA is more virulent than other strains of MRSA such as identified hospital-associated strains, in part through resistance to neutrophil killing. Palazzolo-Ballance et al. (p. 500 ) determined that MRSA evades killing by up-regulating the transcription of genes important for survival upon exposure to the neutrophilic antimicrobial products HOCl, H₂O₂, and azurophilic granule proteins. Exposure to azurophilic granule proteins increased the production of mRNAs encoding several hemolysin and toxin products, explaining the enhanced ability of CA-MRSA to lyse neutrophils. Bacterial genes involved in stress responses were up-regulated after exposure to any of the anti-microbial compounds, but H₂O₂ specifically increased transcripts related to heme/iron uptake such the as iron-regulated surface determinants (isd) isdA and isdB. A strain deficient in these transcripts, ΔisdAB, was more susceptible than wild type to killing by neutrophils, although this could be reversed by pretreatment with an NADPH oxidase inhibitor. Because phagocytosis of both the wild-type strain and the mutant strain was similar, killing was determined to be due to oxidant susceptibility. The authors thus conclude that CA-MRSA has several mechanisms effective for evading neutrophil killing, including direct lysis and oxidant resistance.

Upon CD28 costimulation of T cells, IL-2 is up-regulated via both increased transcription and mRNA stabilization. NF90, an AU-rich element binding protein, is known to translocate from the nucleus to the cytoplasm and stabilize IL-2 mRNA, but how this process is triggered has been unclear. To further clarify the picture of CD28 costimulation and subsequent IL-2 production, Pei et al. (p. 222 ) analyzed the potential regulation of NF90 by phosphorylation. Analysis of the NF90 sequence identified Ser⁶⁴⁷ as a potential Akt family phosphorylation site, and mutation of this residue to Ala eliminated NF90 nuclear export after T cell activation. It was confirmed that Akt kinase could phosphorylate Ser⁶⁴⁷ of NF90 both in vitro and in vivo, and this phosphorylation was found to be critical for IL-2 mRNA stabilization. Further analysis showed that signaling through CD28 induced Akt phosphorylation, which, in turn, phosphorylated NF90 and increased IL-2 protein production. This meticulous study clearly details a CD28-induced mechanism of IL-2 mRNA stabilization and identifies one pathway by which costimulation enhances T cell activation.

It has long been known that the common cold, caused by respiratory RNA viruses, worsens airway inflammation and bronchial responses in asthmatic patients. Shiraishi et al. (p. 541 ) have demonstrated why this occurs. Administering dsRNA in the form of polyinosine-polycytidylic acid (poly I:C) to rats with OVA-induced asthma, the authors showed that airway eosinophilia, bronchial responsiveness, and cyclooxygenase-2 (COX-2)-dependent PGD₂ expression all increased. COX-2 specific inhibitors blocked the alveolar macrophage PGD₂ production that occurred after lung administration of poly I:C. Exposure to dsRNA also increased the Th1 cytokines IFN-γ and IL-12 in the bronchial alveolar lavage (BAL) fluid of OVA-exposed rats when compared with OVA exposure alone. Direct administration of PGD₂ increased lung eosinophilia in OVA-exposed rats but not in animals treated with PBS. This effect could be nearly eliminated by ramatroban, a dual receptor antagonist for the PGD2 receptor (CRTH2) and the thromboxane A₂ receptor (TP), but not by antagonists to TP receptors alone. Poly I:C had no effect on allergen-induced airway eosinophilia in CRTH2-deficient mice, with CRTH2^−/− and wild-type mice showing similar levels of airway eosinophils after allergen exposure. The authors have provided an elegant explanation of how respiratory RNA viruses make asthma worse through the production of PGD₂ and point to CRTH2 as a therapeutic target to ameliorate this condition.

All trans retinoic acid (ATRA) is known to affect hemopoietic stem cells (HSCs), specifically by promoting maturation in the myeloid lineage. Chen et al. (p. 138 ) have expanded on this knowledge to show that ATRA stimulates HSC progression to progenitor cells that mature into B cells and a subset of myeloid cells. Mice that were treated with ATRA had larger spleens when compared with controls, and this increase was due to increased B cell numbers as T and NK cell numbers were unchanged. CD11c⁺CD11b⁺ myeloid dendritic cells were also elevated in the spleens of ATRA-treated mice; however, the total number of CD11b⁺ cells remained the same, indicating a preferential expansion of this subset. In the bone marrow, HSC numbers were normal, the numbers of early lymphoid progenitors (ELPs) and prolymphocytes were significantly reduced, and pre-B cells were decreased. However, numbers of bone marrow CD19⁺B220⁺ and CD45R/B220^Lo IgM⁺ cells increased, indicating that ATRA accelerated the differentiation of progenitors into B cells. In vitro, ATRA was able to reduce maturation time of ELPs to B cell progenitors from 10 days to 7. This work enhances the understanding of how ATRA affects B cell hemopoiesis and development, a matter of particular interest because retinoids are used therapeutically and vitamin A deficiency is linked with immunodeficiency.

Interleukin-15, a proinflammatory cytokine, has been shown to increase the proliferation of CD8⁺ T cells and NK cells and enhance lymphocyte homing to the peripheral lymph nodes (LNs). Mueller et al. (p. 350 ) looked at the action of this cytokine in SIV infection. Macaques acutely infected with SIV were treated with IL-15, resulting in a significant increase in SIV-specific CD8⁺ T and NK cells that correlated with peak viremia. However, the viral set point at week 20 after infection was 3 logs higher in IL-15 treated animals when compared with controls and led to accelerated disease. This may in part be due to an increase in the numbers of Ki-67⁺ CD4⁺ T cells in IL-15 treated animals, thereby creating a larger target population for SIV infection. The anti-SIV Ab response was reduced in animals that received IL-15. The lymph nodes of IL-15-treated macaques had fewer SIV-infected cells at peak viremia compared with untreated SIV-infected animals; however, this was reversed when the viral set point was reached. While SIV-specific CD8⁺T and NK cells were increased by treatment with IL-15, the peak viremia in acute infection remained the same but the viral set point was increased. These data show that although IL-15 treatment increases the number of virus-specific CD8⁺ T cells early in disease, it does not protect against later viremia and increases the overall viral burden.

Tumors can evade immune detection even when expressing Ag. To find out how, Lengagne et al. (p. 130 ) have used a spontaneous mouse model of melanoma to examine what role CD8⁺ T cells play in controlling tumor formation and metastases. Mice transgenic for the human RET oncogene and the chimeric MHC molecule AAD (α₁-α₂ domains of HLA-A2 linked to the H2-D^d α₃ domain) were used to look at CD8⁺ T cell responses to previously identified human melanoma Ags. Eighty-five percent of metallothionein (MT)-ret/AAD mice developed spontaneous tumors at 4 mo and anti-tumor T cell responses were found in all tumor-bearing mice. In vitro, CD8⁺ T cells from the spleens of tumor-bearing mice produced IFN-γ in response to B16.F10 melanoma cells expressing AAD (B16AAD) but not to B16 melanoma cells without AAD, indicating that the anti-tumor response was HLA-A2 restricted. The authors showed that each cutaneous tumor had a unique melanoma differentiation Ag (MDA) profile, but this did not affect the tumor’s ability to elicit Ag-specific CD8⁺ T cell responses in vitro. In contrast, visceral metastases had reduced expression of AAD and MDAs and thus were poorly antigenic in culture. MT-ret/AAD mice were treated with anti-CD8 depletion Ab at 8–11 days of age, before the formation of tumors, to see whether CD8⁺ T cells controlled tumor development. Depletion of CD8⁺ T cells increased visceral metastasis and substantially shortened life span, with >70% of treated mice dead at 6 mo compared with >90% of control mice still alive. Thus CD8⁺ T cells did not change the onset of disease but substantially delayed mortality and the spread of visceral metastases.

Summaries written by Kira R. Gantt, Ph.D.