Thymic stromal lymphopoietin and asthma
Increased production of Th2-type cytokines occurs in asthmatic bronchial mucosa. Thymic stromal lymphopoietin (TSLP) is an IL-7-like cytokine that induces production of Th2-attracting chemokines. However, the level of TSLP in the bronchus is unknown. Ying et al. (p. 8183 ) obtained endobronchial biopsies from 20 asthmatic patients and 15 normal controls. Elevated mRNA expression of TSLP, the Th2-attracting chemokines CCL17 and CCL22, and the Th1-attracting chemokine CXCL10 was seen by in situ hybridization in epithelium and submucosa of samples from asthmatics compared with normal controls; expression of a second Th1-attracting chemokine, CXCL11, was not elevated. TSLP and CCL17 mRNA levels correlated inversely with forced expiratory volume. Significantly higher numbers of major basic protein+ eosinophils and elastase+ neutrophils were detected by immunohistochemistry in bronchial submucosa, but not bronchial epithelium, of asthmatics vs controls. Asthmatics also had higher percentages of CCR4+CD4+ T cells, which correlated inversely with forced expiratory volume and positively with numbers of cells expressing CCL17 or CCL22 mRNA. TSLP was found on cytokeratin+ epithelial cells and CD68+ macrophages in the epithelium, whereas CCL17 and CXCL10 were found on cytokeratin+ epithelial cells and elastase+ neutrophils. In the asthmatic submucosa, TSLP, CCL17, and CCL22 were associated with elevated levels of CD31+ endothelial cells, elastase+ neutrophils, tryptase+ mast cells, and CD68+ macrophages. This is the first comparative study of TSLP expression in the asthmatic bronchus and delineates the cells expressing TSLP and Th1- and Th2-attracting chemokines.
Retinoic acid as an immunoadjuvant
Ross and colleagues have shown that a combination of retinoic acid (RA) and polyriboinosinic:polyribocytidylic acid (PIC) boosts anti-tetanus toxoid (TT) Ab responses in rats lacking vitamin A. In their current study on p. 7961 , Ma et al. looked at primary and secondary immune responses to RA and/or PIC given orally to TT-immunized vitamin A-sufficient mice. PIC alone or RA plus PIC elevated IgM levels 4-fold at 7 days after TT immunization; IgG was elevated significantly at 11 days by RA or PIC alone and >80-fold by RA plus PIC. IgG isotypes were regulated differentially by RA and/or PIC. The type 2/type 1 cytokine mRNA ratio was elevated in spleen cells at day 11 by TT reimmunization. CD8+ T cells at 3 days were significantly reduced by PIC, whereas RA increased NKT cell numbers and PIC increased NK cell numbers. CD11b expression was up-regulated and expression of CD80 was induced on macrophages by RA or RA plus PIC; PIC alone induced CD86 expression. RA and/or PIC given during TT reimmunization enhanced secondary anti-TT IgG production, and RA or PIC differentially regulated the IgG isotypes of splenic Ab-secreting cells. The data show that RA, PIC, or RA plus PIC can direct the type 1/type 2 primary and secondary responses in vitamin A-sufficient mice immunized with TT and suggest that RA plus PIC may improve vaccine performance in the healthy population.
Antiviral responses during persistence
Virus-specific CD8+ T cells hold persistent viral infections in check. However, the evolution of the function and phenotype of antiviral CD8+ T cells from acute to persistent infection has not been examined in detail. Kemball et al. (p. 7950 ) identified three H-2b-restricted polyoma virus (PyV)-specific CD8+ T cell epitopes that stimulated IFN-γ production by spleen cells from acutely infected C57BL/6 mice, a strain resistant to PyV-induced tumors. The epitopes were derived from two nonstructural viral proteins. Expansion-contraction profiles established with either free peptide or individual Db tetramer-peptide complexes were similar only for two of the three peptides during the acute phase; magnitude differences narrowed for all three during persistency. Peptide-specific CD8+ T cell response was Kb and Db restricted in acutely infected mice, but only Kb-restricted cells persisted for the long term. A decrease in Vβ repertoire was seen for splenic PyV epitope-specific CD8+ T cells from acute infection to the persistent phase. Phenotypic differences, as determined by expression patterns of surface molecules and cytokine expression patterns, varied among the three peptide-specific T cell populations during both acute and persistent infections. A novel microchimerism protocol demonstrated that naive bone marrow cells transferred to mice with a persistent PyV infection generated CD62LhighCD8+ T cells, whereas host cells were CD62Llow; both host and donor cells of mice made chimeric before virus infection became CD62Llow. The authors conclude that epitope-specific antiviral CD8+ T cell responses evolve during progression from acute to persistent PyV infection.
IgA1 hinge length and protease susceptibility
Infection at mucosal surfaces is facilitated by IgA1 proteases secreted by a variety of pathogenic bacteria. Although it is known that the proteases cleave the IgA hinge at proline (Pro)-serine or Pro-threonine bonds, other specific features that render the molecule susceptible to cleavage are unknown. Senior and Woof (p. 7792 ) created a series of mutants of a hybrid human IgA2/A1 half hinge Ab, in which one of the duplicated regions of IgA1 protease-sensitive IgA1 (8 aa) was incorporated into the equivalent position of IgA1 protease-resistant IgA2 (total hinge size, 15 aa). IgA1 metalloproteases of all but three tested strains of Streptococcus cleaved the half hinge Ab but not five mutated half hinge Abs. IgA1 proteases from Haemophilus influenzae and two Neisseria species cleaved the half hinge Ab and all mutated forms except one lacking four Pro at the C terminus. A mutant lacking only two Pro was cleaved. Replacement of the deleted four Pro with four alanine residues at the C terminus or addition of four Pro residues at the N terminus restored susceptibility to IgA1 proteases from Haemophilus and Neisseria. Resistance to all but one of the IgA1 proteases (N. gonorrhoeae) was seen with a hinge region reduced to 9 aa. The authors conclude that an IgA1 hinge length between 9 and 12 aa is the minimal size required for cleavage by IgA1 proteases from several pathogenic bacteria.
Toxoplasma gondii persistence
Encysted bradyzoites (BZs) of the intracellular, protozoan parasite Toxoplasma gondii convert to tachyzoites (TZs) after ingestion by a host. TZs that survive the host immune response disseminate primarily to the brain where they convert back to BZs, encyst, and establish a chronic infection. It is not known whether stage-specific expression of surface Ags is a mechanism for BZs to attain persistency. Kim and Boothroyd (p. 8038 ) developed mutant parasites that constitutively expressed either a BZ stage-specific Ag (SRS9c) or a TZ stage-specific Ag (SAG1c). CBA/J mice infected with 4000 TZs of SRS9c mutants had slightly increased survival and fewer brain cysts over time compared with mice infected with wild-type TZs. Mice infected with 10-fold fewer SRS9c TZs had improved survival and a similar number of brain cysts during the chronic phase. Infection with SRS9c, but not wild-type, TZs resulted in high-titer SRS9-specific IgG and an SRS9-specific splenic T cell response characterized by IFN-γ production. Fewer brain cysts were seen early in mice infected with SAG1c TZs compared with mice infected with wild-type TZs; however, there was an increase in both the number of brain cysts and mortality in the SAG1c TZ-infected animals during the late phase of the chronic infection. SAG1 protein-activated splenocytes from SAG1c TZ-infected mice produced IFN-γ as well as TNF-α and IL-10, and mononuclear infiltrates were greater in these mice than in mice infected with SRS9c or wild-type TZs. The data demonstrate that a TZ to BZ antigenic shift allows T. gondii to escape immune surveillance and to achieve persistency in the infected host.
Understanding delayed xenograft rejection
Delayed xenograft rejection (DXR) is a major obstacle to the use of xenografts for treatment of organ failure. Although DXR is characterized by monocyte accumulation within the xenograft, the mechanism responsible for that accumulation is unknown. Peterson et al. (p. 8072 ) found that human monocytes adhered more firmly to porcine aortic endothelial cells (PAEC) than to human aortic endothelial cells (HAEC) in a parallel plate flow chamber. L-selectin was the only adhesion molecule expressed at a higher level on unactivated PAEC compared with HAEC. Monocyte binding to PAEC did not occur in calcium-free perfusion buffer; anti-α4 integrin Ab plus anti-β2 integrin Ab completely blocked cell adhesion, whereas anti-L-selectin Ab had no effect. Galactosidase treatment to remove the terminal sugar residues from galactoseα(1,3)galactoseβ(1,4)GlcNAc-R (α-gal) expressed on xenogeneic endothelium, or specific binding of isolectin to α-gal, reduced adhesion of monocytes to PAEC. Most monocytes exhibited calcium-dependent binding of a soluble form of α-gal-FITC conjugate but did not bind to plates coated with α-gal. Increased monocyte adhesion and greater resistance to high shear stress were seen on α-gal-BSA co-immobilized with ICAM-1 or ICAM-2, than on BSA co-immobilized with either ICAM. Soluble α-gal binding to monocytes resulted in activation of β2 integrins to a higher affinity state. The data indicate that α-gal on PAEC, but absent from HAEC, binds a C-type lectin on the monocyte surface and up-regulates β2 integrin expression to result in enhanced monocyte adhesion.
Regulation of ROS in adherent neutrophils
Suppression of NADPH oxidase activity in adherent neutrophils migrating through tissues to inflammatory sites is essential to prevent excessive production of reactive oxygen species (ROS) and tissue damage. But activation of NADPH oxidase and ROS formation at the site of infection are required to kill infecting microbes. Zhao and Bokoch (p. 8049 ) treated human neutrophils in suspension with TNF-α before immobilization on fibronectin and activation by fMLP. The treatment overcame the 30- to 60-min lag in H2O2 production seen in adherent neutrophils not exposed to TNF-α; it also induced H2O2 production and phosphorylation of proline-rich tyrosine kinase 2 (pyk2) equivalent to that seen for fMLF stimulation of neutrophils in suspension. Similar results were seen with neutrophils exposed to GM-CSF or platelet-activating factor before fMLP stimulation. Treatment of adherent neutrophils with a pyk2 kinase inhibitor or introduction of a dominant-negative pyk2 mutant protein inhibited pyk2 activation after TNF-α pretreatment. TNF-α pretreatment also reversed adhesion-mediated suppression of the cytosolic NADPH oxidase component Rac2 and its upstream guanine nucleotide exchange factor, Vav1, and reduced tyrosine phosphatase activity before and after stimulation. Inhibition of tyrosine kinase syk resulted in loss of Vav1 activation in adherent neutrophils treated with TNF-α and fMLP. The authors suggest that pretreatment of adherent neutrophils with cytokines or chemotactic factors activates a pyk2 signaling pathway that reverses adhesion-mediated suppression of ROS formation.
Summaries written by Dorothy L. Buchhagen, Ph.D.