A placental IgG transporter
Transport of IgG across the placental syncytiotrophoblast is mediated by FcRn, an MHC-related FcR for IgG. IgG is thought to cross a second placental layer, the villus endothelium, passively in transcytosing caveolae. Takizawa et al. (p. 2331 ) stained ultrathin cryosections of human placental terminal villi with Abs to cell components and examined them by immunofluorescence and immunoelectron microscopy. FcγRIIb2, another FcR for IgG, was detected exclusively in endothelial cells within the placental endothelium using two anti-FcγRIIb Abs. FcγRIIb did not colocalize with any of 17 other known marker proteins, including one specific for caveolae, in double-label immunofluorescence microscopy. FcγRIIb-positive structures were found primarily in the intracellular portion of the placental endothelial cells; by contrast, caveolae were found primarily at luminal and abluminal surfaces. IgG was at highest concentration in the extracellular matrix in placental terminal and intermediate villi, but it also was detected in the cytoplasm of the syncytiotrophoblast and endothelium. Approximately 40% of FcγRIIb-positive punctate structures within the cytoplasm of placental endothelial cells contained IgG, whereas 80% of the compartments that contained IgG also contained FcγRIIb. Most of the IgG was found at or near luminal and abluminal plasma membranes. This analysis of ultrathin cryosections of placental villi identifies a novel FcγRIIb-containing organelle that contains IgG and may mediate its active transfer across the endothelium to the fetus.
ICOS and allergies
Although ICOS, a member of the CD28 family, is associated with Th2-mediated diseases such as asthma and allergy, no definitive evidence of that linkage has been demonstrated in humans. Shilling and colleagues (p. 2061 ) found several single-nucleotide polymorphisms (SNPs) within the ICOS gene among the inbred Hutterite population. Two SNPs within the 5′ flanking sequence associated significantly with allergic sensitization to airborne allergens and increased total serum IgE levels. The association was greatest in individuals homozygous for the two SNPs. One SNP was within a site 90% homologous to the consensus NF-κB p50 binding site. Binding of proteins in nuclear extracts from cultured human cells to radiolabeled SNP or NF-κB oligonucleotides was competed by either unlabeled oligonucleotide. Anti-p65 Ab caused a supershift of the oligonucleotide-protein complexes; a greater supershift was seen with anti-p50 Ab. CD4+ T cells from non-Hutterite individuals homozygous for the SNP mutation had elevated ICOS expression before and after activation, and produced higher levels of IL-4, IL-5, IL-13, and TNF-α after activation, compared with CD4+ T cells from individuals that did not carry that SNP. The data demonstrate a direct association of a mutation within the 5′ promoter of ICOS with Th2-mediated allergic sensitization.
Evading an antimalarial vaccine
Although malaria is prevalent in several areas of the world, individuals in the endemic regions develop only partial parasite immunity with predominately IgM responses. Wykes et al. (p. 2510 ) determined that mice immunized with the C-terminal fragment of the merozoite surface protein-1 (MSP119) of Plasmodium yoelii generated high titers of vaccine-specific IgG Ab and were protected against parasite infection 2 wk later compared with unvaccinated controls. By 10 wk, vaccinated mice boosted with MSP119 developed increased vaccine-specific IgG titers and survived infection but developed parasitemia. Memory spleen cells transferred from immunized mice into naive recipients produced Ab to MSP119 after boosting but did not confer any greater protection against infection in SCID mice than cells from PBS-immunized mice. Mice given hyperimmune anti-MSP119 serum cleared their parasitemia. A significant reduction in the number of vaccine-specific Ab-secreting cells was found by ELISPOT assay in sublethally irradiated naive mice that had been given MSP119-specific memory cells and then infected with P. yoelii compared with controls boosted with MSP119. By flow cytometry, 6-fold more memory B cells were present in splenocytes from immunized mice following boosting, whereas 6-fold more memory B cells underwent apoptosis following infection. Infection increased apoptosis of long-lived plasma cells, resulting in a reduction of >50% compared with boosted controls. Apoptosis of long-lived plasma cells, but not memory cells, specific for two other immunogens also was increased by parasite infection. The authors conclude that P. yoelii circumvents long-term protection from vaccines in mice by increasing apoptosis of IgG memory B cells and long-lived Ag-specific plasma cells.
NKT cells and allograft tolerance
Chemokine receptor CXCR6 is expressed on NKT cells. Although NKT cells maintain peripheral transplantation tolerance induced by costimulation blockade, there is no evidence that CXCR6 is directly involved. Jiang et al. (p. 2051 ) found that murine cardiac transplant recipients treated with anti-CD40L mAb plus a mAb against CXCL16, the CXCR6 ligand, had 100-day allograft survival comparable to that in anti-CD40L mAb-treated NKT−/− controls (27%); allograft survival in recipients given only anti-CD40L mAb was 91%. Expression of CXCL16 mRNA and NKT cell numbers in harvested cardiac allografts were high in anti-CD40L mAb-treated mice, intermediate to low in recipients treated with anti-CD40L mAb plus anti-CXCL16 mAb, and very low in untreated syngeneic recipients. Cardiac grafts from all but the syngeneic recipients were positive for CXCL16 protein by immunohistochemistry. In vitro, NKT cells selectively attached to a monolayer of CXCL16 transgenic cells expressing the chemokine on cell membranes; attachment was prevented by the addition of anti-CXCL16 mAb. The data indicate that CXCL16/CXCR6 interaction within an allogeneic cardiac transplant regulates NKT cell trafficking and NKT cell-mediated transplant tolerance.
Rejuvenating the aged thymus
Decline in thymic structure and function with age can be reversed in both mice and humans by surgical or chemical castration. However, there is no detailed study of restored thymic functions following castration. Sutherland et al. (p. 2741 ) compared several thymic parameters in young adult mice (2 mo old) and aged mice (24 mo old) that were either untreated or surgically castrated. Aged thymi were smaller and had a disrupted microenvironment with decreased expression of MHC class II and enhanced expression of Ags specific for vascular fibroblasts, extracellular matrix proteins, and thymic epithelium compared with young thymi. Following castration, all of these parameters were reversed to levels seen in young adult mice. In thymi of aged mice, castration restored to levels seen in young adult mice features associated with aging including greatly reduced T cell proliferation, increased apoptosis, greater accumulation of CD44+CD25− T cells, and reduced proportion of c-kit+ cells. Castration also corrected the lower export to the periphery of CD4+ and CD8+ T cells with reduced proliferative capacity in aged mice. Castrated young adult recipients had increased thymus cellularity of donor origin by 4 wk after receiving bone marrow transplants compared with sham-operated controls; similar results were seen in recipients chemically castrated using a luteinizing hormone-releasing hormone agonist. Sixteen prostate cancer patients older than 60 years had increased numbers of peripheral naive CD4+ and CD8+ T cells after undergoing standard combined androgen blockade before localized radiation therapy. These studies demonstrate that surgical or chemical castration in mice and chemical castration in humans restores thymic functions.
Surfactant protein A and macrophages
Schlesinger and colleagues showed that surfactant protein A (SP-A), a primary component of pulmonary surfactant, increases macrophage phagocytosis by enhancing activity of the mannose receptor (MR). However, little is known about the signaling pathways induced by SP-A in human macrophages. In work from the same laboratory, Beharka et al. (p. 2227 ) measured a significant increase in cytosolic Ca2+ concentration in human monocyte-derived macrophages (MDM) in response to alveolar proteinosis protein (APP) SP-A; responses to other sources of SP-A were not as dramatic. The response was concentration dependent and induced a state refractory to a second stimulation with SP-A for up to 1 h. Stimulation experiments with mutated recombinant rat SP-A proteins demonstrated a requirement for both carbohydrate moieties and the collagen-like domain for maximum Ca2+ mobilization. A loss of the APP SP-A-induced Ca2+ response occurred in MDM pretreated with an inhibitor of endoplasmic reticulum Ca2+-ATPase. Intracellular levels of d-myo-inositol 1,4,5-triphosphate, formed through phospholipase C hydrolysis of membrane phospholipids and important in regulating Ca2+ release from the endoplasmic reticulum, directly correlated with APP SP-A concentration. Pretreatment of MDM with a PI3K inhibitor significantly decreased the cytosolic Ca2+ response. SP-A-induced up-regulation of MR expression on MDM was reduced by preincubation with either of two PI3 inhibitors or with an inhibitor of extracellular Ca2+ influx. These studies indicate that SP-A induces MR expression on MDM by activating a signal transduction pathway involving PI3K and Ca2+.
APC ICAM-1 and central memory T cells
Formation and maturation of the immunological synapse between an APC and a T cell is influenced by ICAM-1/LFA-1 interaction. Yet the impact of ICAM-1 on the generation of memory cell subsets against pathogens is not known. Parameswaran et al. (p. 2201 2201) found that modulation of activation markers on splenocytes from ICAM-1−/− mice after stimulation in vitro with anti-CD3 Ab was less pronounced than on splenocytes from wild-type mice. Reduced proliferation of ICAM-1−/− splenocytes in response to low doses of anti-CD3 Ab was partially restored by exogenous IL-2, and fully restored by anti-CD28 Ab, to levels seen in control cultures. CD8+ T cells from mice transgenic for a viral peptide-specific TCR were activated by peptide-pulsed wild-type, but not ICAM-1−/−, peritoneal exudate cells. Wild-type CFSE-labeled T cells proliferated poorly in sublethally irradiated ICAM-1−/− allogeneic hosts, whereas both mutant and wild-type cells proliferated in wild-type recipients. Mutant T cells, poorly activated by anti-CD3 Ab in the presence of wild-type or mutant splenocytes, had increased activation levels in mixed cultures of wild-type and mutant splenocytes or at increased mutant T cell density. OVA or live bacteria stimulated equivalent numbers of effector CD4+ T cells in both strains of mice, but decay of both the cytokine response and antibacterial immunity was rapid only in the mutant mice. Mutant mice were unable to mount a detectable T cell response when immunized with OVA peptide. The ratio of central to effector memory T cells was lower in naive ICAM-1−/− mice than in wild-type mice. The results suggest that in vivo differentiation of activated T cells into central memory cells requires signals provided by ICAM-1 on APCs.
Summaries written by Dorothy L. Buchhagen, Ph.D.