Limiting C damage
Preventing the formation of the C3 convertase reduces damage from inflammation following activation of the mammalian C classical pathway. Schistosoma complement C2 receptor inhibitor trispanning (CRIT) is known to bind C2 and prevent its cleavage. Inal et al. (p. 356 ) found CRIT homologues in genomic DNA from Trypanosoma, human, rat, and cod by Southern blotting and PCR. Ab against an extracellular N-terminal peptide enabled detection of CRIT on the surface of Jurkat cells by flow cytometry. CRIT expression in a wide range of human tissues and cells was shown using Abs against the N-terminal peptide and other regions of the protein. The binding of biotinylated C2 to filter-immobilized CRIT or of fluorescence-labeled C2 to Jurkat cells was abolished by preincubation of the filter or cells with anti-N-terminal peptide Ab; biotinylated C2 interaction with C4 or C4b was blocked by the same Ab, by CRIT peptides, or by C4 peptides. Anti-CRIT-terminal peptide Ab increased C lysis of CRIT+ human cells sensitized with anti-lymphocyte Abs. Jurkat cells or the CRIT N-terminal peptide inhibited C1s-mediated cleavage of C2 protein in vitro, but C2 cleavage occurred after incubation of Jurkat cells with anti-N-terminal peptide Ab. The authors conclude that CRIT, most likely acquired from parasites via horizontal transmission, binds C2 to prevent C3 convertase formation and limit inflammation damage to tissues.
CXCR6 and CD8+ T cell homing
Accumulation of donor CD8+ T cells in the liver of a transplanted host occurs during graft-vs-host disease. Although the activated cells express high levels of chemokine receptor CXCR6, it is not known whether CXCR6 is directly involved in T cell recruitment. Sato et al. (p. 277 ) found that liver-infiltrating CD8+ and CD4+ T cells from mice in which GFP replaced one CXCR6 allele had increased liver accumulation and higher receptor mRNA expression under inflamed (allogeneic transfer) conditions vs normal (syngeneic transfer) conditions. Expression of the ligand CXCL16 mRNA also was up-regulated in the inflamed liver. Fewer donor CD8+ T cells were found in the inflamed liver 6 h or 7 days after allogeneic transfer of cells from mice carrying GFP knockins at both CXCR6 alleles; no difference in donor CD4+ T cell numbers or percentages was seen. These CD8+ T cell reductions were found to be due to reduced recruitment to the liver rather than decreased proliferation after recruitment by measuring infiltrating donor cells 6 h after injection. CXCR6 deficiency did not alter levels of donor CD8+ T cells in the blood or spleen compared with wild-type donor cells. The data support a specific role for CXCR6 in homing of activated donor CD8+ T cells to the inflamed liver in graft-vs-host-induced hepatitis.
TLR enhancement of diabetes
Kilham rat virus (KRV) infection of BBDR rats results in type 1 diabetes in ∼25% of the animals. The frequency is increased to 100% in rats pretreated with the TLR3 agonist poly(I:C). To investigate the role of TLR-induced activation of innate immunity in autoimmune diabetes induction, Zipris et al. (p. 131 ) found that pretreatment of BBDR rats with other TLR ligands, including heat-killed bacteria, also enhanced KRV-induced diabetes induction; some ligands were effective at low viral doses that were not usually diabetogenic. Treatment with poly(I:C) before low-dose virus infection resulted in higher serum levels of virus-specific Abs compared with low-dose virus infection only. Serum levels of IL-12 p40 were increased in KRV-infected BBDR rats, compared with KRV-infected diabetes-resistant WF rats, but not in BBDR rats infected with nondiabetes-inducing viruses or in uninfected controls. Expression of IL-12 p40, IP-10, and IFN-γ mRNAs were higher in pancreatic, cervical, and mesenteric lymph nodes from KRV-infected BBDR or WF rats compared with uninfected and vaccinia virus-infected controls. The IP-10 protein level in pancreatic lymph nodes of KRV-infected BBDR rats was higher than in vaccinia virus-infected rats. The authors conclude that TLR activation of the innate immune system before infection with diabetes-inducing KRV increases disease incidence in BBDR rats.
CD28 regulation of T cell survival
CD28 costimulation of TCR-activated T cells results in Bcl-XL up-regulation and increased cell survival. However, the mechanism by which CD28 acts is unknown. Wu et al. (p. 180 ) found that anti-CD3/anti-CD28 mAb costimulation reduced activation-induced cell death in wild-type mouse T cells but not in CD28−/− T cells or in T cells from transgenic mice carrying a CD28 mutation unable to recruit PI3K. Flowcytometry showed thatBcl-XL protein levels, but not Bcl-2 protein levels, were increased in wild-type cells even in the presenceof anti-IL2 mAb; inhibitors of either PI3K or target of rapamycin (mTOR) kinase activity reduced the costimulation-induced Bcl-XL protein expression. Either anti-CD3 mAb or anti-CD3/anti-CD28 mAbs stimulated Bcl-XL mRNA levels equally in wild-type and CD28−/− mice. Costimulation also inactivated the repressor of cap-dependent translation. Primary mouse T cells and an established human T cell line transfected with a luciferase reporter gene under control of Bcl-XL 5′ untranslated nucleotides exhibited enhanced luciferase activity after costimulation. Luciferase activity was abolished by cotransfection with a vector expressing a negative regulator of protein synthesis downstream of mTOR or an antagonist of PI3K. Bcl-XL mRNAs in costimulated wild-type cells were found in the polyribosome pool, whereas those in costimulated CD28−/− cells were distributed equally between the free and polyribosome pools. The authors conclude that anti-CD28 costimulation of activated T cells prevents apoptosis by relieving inhibition of Bcl-XL mRNA translation through activation of the PI3K/mTOR pathway.
Plasmacytoid dendritic cell migration
Plasmacytoid dendritic cells (pDCs) migrate from blood to sites of inflammation. Yet, unlike myeloid DCs (mDCs), they do not respond to inflammatory chemokines. To identify a chemokine receptor and its ligand specific to pDCs, Zabel et al. (p. 244 ) developed a mAb to a human candidate receptor chemokine-like receptor 1 (CMKLR1) expressed by DCs generated in vitro from monocytes. The mAb reacted with a subset of circulating DCs with morphology similar to pDCs but not with circulating T cells, B cells, NK cells, or mDCs. CpG oligonucleotide activation of pDCs resulted in down-regulation of CMKLR expression. Chemotaxis of CMKLR1-transfected cells in response to human serum fractions led to the identification of the protein product of the tazarotene-induced gene 2, or chemerin, as the chemotactic agent. A variety of tissues and organs, most notably liver, pancreas, and adrenal gland, expressed chemerin mRNA. Serum had more chemerin attractant activity than plasma. Recombinant chemerin induced migration of pDCs, but not mDCs, across monolayers of human umbilical vein endothelium, and chemerin in conditioned medium attracted pDC in Transwell chemotaxis assays. The authors speculate that chemerin in the blood is an inactive proform that is processed to an active form by proteases at sites of inflammation or tissue damage to which the ligand recruits CMKLR1-expressing pDCs.
CTLA-4 and food allergies
Although incidence of food allergies, especially to peanuts, is increasing, mechanisms inducing sensitization are not known. Several lines of evidence suggest that CTLA-4 signaling has a role in tolerance induction. van Wijk et al. (p. 174 ) orally exposed mice to peanut protein extract (PPE) with or without the mucosal adjuvant cholera toxin (CT) in a mouse model of peanut sensitization. Mice treated with PPE plus CT had PPE-specific serum IgG1, IgG2, and IgE, whereas mice treated with PPE alone had only low levels of PPE-specific IgG1 and IgG2; IgE was reactive with peanut allergens Ara h 1, Ara h 3, and Ara h 6. Anti-CTLA-4 Ab administered during oral sensitization enhanced the PPE-specific Ab levels in the group receiving PPE plus CT but not in the group receiving PPE only. Anti-CTLA-4 Ab treatment increased total IgE levels in both PPE-exposed groups. Serum levels of mast cell degranulation products were elevated only in PPE-challenged mice sensitized with PPE plus CT; anti-CTLA-4 Ab exacerbated the response. In vitro treatment with anti-CTLA-4 Ab of mesenteric lymph node and spleen cells isolated from animals 4 wk after PPE treatment resulted in release of higher levels of IL-4, IL-5, and IL-10; there was no difference between cells cultured with or without PPE. In contrast, elevation of IFN-γ occurred in all anti-CTLA-4 Ab-treated groups, but was significantly increased in cells from animals treated with PPE plus CT. The authors conclude that CTLA-4 signaling is not the crucial factor in preventing sensitization to food proteins but regulates the intensity of the allergic response.
Preventing burn wound infection
One approach to preventing opportunistic infections, and associated mortality, in severe burn injury patients is to stimulate production of new immune cells. In a mouse model of Pseudomonas aeruginosa burn wound infection, Toliver-Kinsky et al. (p. 404 ) found 86% mortality in animals inoculated with bacteria immediately after injury. Mortality was reduced to 10% in mice that had received the hemopoietic cytokine Fms-like tyrosine kinase-3 ligand (Flt3L) daily for 5 days before injury and infection. By 48 h after burn wound inoculation, Flt3L-treated mice had lower bacterial counts within the wound compared with mice receiving a control injection. Survival was 95% in mice that received i.p. injection of dendritic cells (DCs) purified from spleens of mice treated with Flt3L 24 h before burn and wound infection. There was little or no improvement in survival rates of controls receiving no DCs, purified NK cells from Flt3L-treated mice, or DC- and NK cell-depleted splenocytes from Flt3L-treated mice. The data indicate that Flt3L treatment before burn wound increases splenic DC numbers, decreases bacterial growth, and reduces mortality.
Summaries written by Dorothy L. Buchhagen, Ph.D.