Various therapeutic strategies have been developed to treat Ab-mediated autoimmune disorders, including treatment with intravenous immunoglobulin (IVIG), which is thought to reduce autoimmune inflammation in part by binding to neonatal FcR (FcRn) at saturating levels. FcRn is an MHC class I-related molecule that mediates transport of IgG molecules across different cellular barriers and protects IgG from intracellular destruction, thus prolonging its half-life. Patel et al. (p. 1015) used the murine serum-transfer model of arthritis to determine if recombinant Abs termed Abdegs (Abs that enhance IgG degradation), engineered to block FcRn by binding to the Fc region, could be used as an alternative to IVIG therapy. Mice treated with Abdegs at various times after arthritis-inducing serum transfers exhibited significantly reduced arthritis symptoms, compared with untreated control mice. FcRn blockade was identified as the primary mechanism mediating the effectiveness of Abdegs, and Abdeg administration did not significantly affect FcRn expression. In addition, a significantly lower dose of Abdegs was effective in reducing arthritis symptoms relative to IVIG treatment. Taken together, these observations provide evidence that FcRn blockade with an engineered Ab may provide an alternative approach to the management of Ab-mediated autoimmune disorders.
New Hint in HIV Hindrance
Infection of macrophages by HIV-1 is known to be modulated by molecules secreted from CD4+ T cells, including chemokines and cytokines. Raposo et al. (p. 748) observed that decreased HIV-1 infection in macrophages was associated with downregulation of their CD4 expression caused by soluble factors released from activated CD4+ T cells. CD4 downregulation was dependent on posttranslational serine phosphorylation as well as intracellular degradation through processes involving proteasomes and vacuolar acidification. In addition, the activities of NF-κB and protein kinase C were required for CD4 downregulation. Cytokines released by Th1- or Th2-polarized T cells, as well as IL-16, RANTES, and macrophage inhibitory factor were ruled out as the soluble factors causing CD4 downregulation. Proteomic analysis identified several candidate proteins in the 50–100-kDa range from the fraction of the activated T cell supernatant showing the greatest effect on reducing HIV-1 infection and CD4 expression. Proteins involved in cell adhesion and spreading were highly represented in this fraction of the supernatant and included attractin, fibronectin, and galectin-3–binding protein. These data suggest that soluble factors from activated T cells limit CD4 expression through multiple mechanisms, thus altering macrophage susceptibility to HIV-1 infection.
CpG Evasion Selection
Detection of alpha- and betaherpesviruses by TLR9 has been shown to be essential to antiviral responses, but the role of TLR9 during gammaherpesvirus infection is less clear. In this issue, Pezda et al. (p. 887) examined TLR9 detection of murine gammaherpesvirus 68 (MHV68). TLR9 molecules in the endolysosomal compartment of dendritic cells (DCs) engaged by viral DNA or CpG dinucleotide motifs can activate DCs and induce IFN-I production. MHV68 did not lytically infect plasmacytoid DCs (pDCs), nor did it induce significant IFN-I production. MHV68 did not appear to interfere with TLR signaling, as pDC responses to the TLR ligands R848 or CpG were not compromised by MHV68 exposure. Taken together, these data suggested that TLR9 was not detecting MHV68. In support of this idea, purified genomic DNA from MHV68 weakly activated TLR9 compared with genomic DNA from the betaherpesvirus murine CMV (MCMV). Analysis of the MHV68 genome revealed significantly fewer CpG motifs relative to the MCMV genome. This underrepresentation of CpG motifs appeared to be caused in part by deamination of 5-methyl cytosines to thymines within CpG dinucleotides and positive selection of viruses with these mutated motifs. These results clarify strategies adopted by gammaherpesvirues to evade TLR9 detection and destruction by innate immune responses.
Osteoclasts are considered key contributors to the bone and joint damage associated with arthritis. Adamopoulos et al. (p. 951) investigated the role of the proinflammatory cytokine IL-23 during arthritis and bone metabolism as well as its effects on osteoclasts. Mice exposed systemically to IL-23 by injection with minicircle DNA encoding this cytokine developed arthritis symptoms and showed increased bone loss, myelopoiesis, and elevated IL-17A serum concentrations relative to control mice. Systemic exposure of mice to IL-17A did not induce arthritis symptoms, but depletion of IL-17A, TNF, or CD4+ T cells in mice with systemic IL-23 expression attenuated arthritis. Mice deficient in the p19 subunit of IL-23 (IL-23p19−/−) are known to be resistant to autoimmune joint destruction. In this study, bone marrow-derived macrophages from IL-23p19−/− mice showed reduced osteoclast maturation and activity compared with bone marrow-derived macrophages from wild-type (WT) mice. Older IL-23p19−/− mice had a slightly higher cancellous bone volume relative to WT mice, indicating abnormal osteoclast activity. These data suggest that systemic IL-23 contributes to arthritis symptoms and promotes osteoclast maturation from myeloid precursors and thus support the exploration of IL-23–targeted therapies.
Nibbling at Tumors
Macrophages and natural killer cells have been shown to be the principal cell types associated with tumor destruction by Ab-dependent cellular cytotoxicity (ADCC), but less is known of the contribution of polymorphonuclear cells (PMNs). Bakema et al. (p. 726) characterized the mechanism by which PMNs may induce Ab-dependent tumor destruction via FcαRI. Using a plate-bound ADCC assay, the authors observed tumor cell killing in an adherent mammary tumor cell line treated with PMNs and a bispecific Ab (BsAb) able to bind to both tumor Ag and FcαRI. Autophagy, and not apoptosis, was found to be a feature of tumor cell killing by PMNs, as microscopic analysis of nonviable cells showed an increase in double membrane-containing vesicles and vacuolization, compared with viable cells. Tumor cells treated with PMNs and FcαRI BsAb showed increased expression of LC3B in autophagosomes, and ADCC-mediated killing was inhibited in the presence of the autophagy inhibitor ammonium chloride. Microscopic evidence of necrotic cell death was also observed. Taken together, these findings indicate that PMNs can kill tumor cells by autophagy in an FcαRI-specific manner, lending support to the development of new tumor therapies targeting this mechanism.
Adaptively tolerant T cells have been described as existing in a hyporesponsive state owing to prolonged Ag exposure and defects in TCR stimulation. Choi and Schwartz (p. 805) investigated the interaction of adaptively tolerant T cells with APCs to better understand the mechanisms contributing to this hyporesponsiveness. Adaptively tolerant T cells formed conjugates with APCs similar to those formed by naive or preactivated T cells. However, immunological synapse formation was significantly defective in adaptively tolerant T cells and correlated with impaired polarization events at the T cell-APC interface, including reduced translocation of ZAP70, LAT, phospholipase C γ1, and PKC-θ. VAV1, critical to initiating actin polarization during synapse formation, was phosphorylated in adaptively tolerant T cells to a degree similar to that in naive or preactivated T cells but was not substantially recruited to the synapse. This defect was linked to reduced expression of ITK (IL-2–inducible T cell kinase) and GADS (growth factor receptor-bound protein 2-related adaptor downstream of SHC). This, along with reduced LAT phosphorylation, resulted in reduced phosphorylation of SLP76 (SRC homology-2–domain containing leukocyte protein of 76 kDa) and VAV1 recruitment to the synapse. These results indicate that impaired immunological synapse formation contributes to the hyporesponsive state of adaptively tolerant T cells.
During polymicrobial sepsis, neutrophils are rapidly mobilized from the bone marrow to the peripheral circulation, where they provide innate defense against bacteria. Delano et al. (p. 911) used the cecal ligation and puncture (CLP) model to understand the mechanisms involved in neutrophil mobilization during sepsis in mice. Mice deficient in MyD88, TRIF, IFNα/βR, or functional TLR4 showed no defect in neutrophil mobilization upon CLP, compared with wild-type mice, indicating that neutrophil release from the bone marrow did not require TLR signaling. Instead, CLP-induced sepsis correlated with changes in CXCL2, CXCR4, and CXCL12 expression in the bone marrow. Blockade of the CXCL12/CXCR4 interaction with anti-CXCL12 antisera significantly decreased bone marrow mobilization and peritoneal accumulation of neutrophils relative to CLP controls during sepsis, but CXCL2 blockade had no effect. Anti-CXCL12 antisera treatment was also associated with decreased bacterial clearance and survival. Of interest, CXCL12 blockade had no effect on hematopoiesis or neutrophil development in the bone marrow. These findings illustrate a role for CXCL12 in neutrophil mobilization during sepsis.
Regulatory T Cell Tinkering
Regulatory T cells (Tregs) in the skin are important for maintaining tolerance at a site where it is essential for the immune system to distinguish self from non-self. The manner in which Tregs deal with autoreactive T cells in the skin is not clear, and Killebrew et al. (p. 861) developed a TCR transgenic mouse to explore this mechanism. Tcra and Tcrb genes from an autoreactive CD4+ T cell clone derived from a Foxp3-deficient scurfy mouse were used to generate SFZ70.Rag1−/− mice expressing a TCR that recognizes a cutaneous autoantigen. The frequencies of both CD4/CD8 double positive and double negative thymocytes in SFZ70.Rag1−/− mice were similar to those in wild-type mice, but SFZ70.Rag1−/− mice had significantly fewer CD4+ single positive T cells. Nonetheless, CD4+Foxp3− conventional T (Tconv) cells and CD4+Foxp3+ Tregs were still detected in the periphery of SFZ70.Rag1−/− mice. In addition, SFZ70 Tregs showed an activated phenotype and expressed the skin homing molecules CD103 and P-selectin ligand. SFZ70 Tregs did not appear to inhibit activation of the autoreactive SFZ70 Tconv cells but did suppress their function by limiting expression of T cell homing molecules and proinflammatory cytokines. In SFZ70.Rag1−/− mice, Tconv cell inhibition by Tregs could be surmounted by anti–CD40-mediated activation of dendritic cells. Thus, SFZ70.Rag1−/−mice serve as a unique model to discern the mechanisms by which Tregs maintain tolerance in the skin.
Summaries written by Christiana N. Fogg, Ph.D.