Parasite versus LPS signaling
Control of the inflammatory response by the intracellular pro-tozoan Toxoplasma gondii is mediated through down-modulation of signaling pathways reminiscent of endotoxin tolerance induced by LPS. But on p. 3003 , Kim et al. show that the two mechanisms are distinct. Mouse macrophages stimulated with LPS 6 h after infection with T. gondii produced less TNF-α and IL-12 than unstimulated cells but more than medium-treated cells; cells pretreated with LPS did not respond to secondary LPS stimulation. The cytokine suppression pattern with T. gondii infection mimicked that seen with an inhibitor of the p38 subunit of mitogen-activated protein kinase (MAPK), although LPS and T. gondii MAPK phosphorylation kinetics were similar. T. gondii pretreatment was more effective than LPS pre-exposure in blocking LPS-induced rephosphorylation of MAPK. LPS-triggered NF-κB translocation did not occur in macrophages pretreated with LPS but did occur in cells acutely or chronically infected with T. gondii. T.gondii-infected, but not unstimulated or LPS-stimulated, cells expressed high levels of Toll-like receptor 4 by 24 h after LPS triggering. The authors suggest that the MAPK inactivation induced by T. gondii infection of macrophages is the result of events downstream of MAPK that differ from those that occur in endotoxin tolerance.
Age-dependent CMV immunity
Cytomegalovirus (CMV)-infected infants and children have fewer CMV-reactive CD4+ T cells and serve as significant reservoirs for transmission of virus to susceptible hosts. Tu et al. (p. 3260 ) found that 13 asymptomatic young children with primary CMV infection had a lower frequency of CMV-specific CD4+ T cells producing IFN-γ and IL-2 and expressing CD154 than 14 asymptomatic chronically infected adults and 4 healthy adults with primary CMV infection. The lag in anti-viral immunity among the children lasted for a year or more following viral infection. All three groups had comparable levels of IFN-γ-producing CMV-specific CD8+ T cells. Memory CD4+ T cells from adults and children were predominantly of the CCR7low subset; the number of those cells was significantly lower in children with primary CMV infection compared with adults with chronic infection. However, the frequency of IFN-γ-producing CD4+ T cells responding in vitro to staphylococcal enterotoxin B was equivalent in all CMV-infected groups. Viral shedding in the urine of children was detected 12–29 mo after infection by real-time PCR assay and culture, whereas adults ceased urinary shedding by 6 mo after primary infection. The authors suggest that children have a selective reduction in the CMV-specific CD4+ T cell response that is inversely correlated with viral shedding.
An IL-23/IL-17 axis
The proinflammatory cytokine, IL-23, consists of a helical subunit, p19, and a soluble receptor subunit, p40; p40 also is found in association with p35 in IL-12. IL-23 engagement with its receptor on CD4+ T cells results in induction of IL-17. To study the relationship between IL-23 and IL-17, Ghilardi et al. (p. 2827 ) used a targeted vector to replace the coding region of p19 with a reporter gene. In contrast to IL-12p40−/− mice, IL-23p19−/− mice were phenotypically normal and had Ig levels, B cell responses to T-independent Ags, B cell proliferation in vitro, and isotype switching comparable to wild-type mice. However, after primary immunization, IL-23p19−/− and IL-12p40−/− mice had lower levels of OVA-specific IgG2a and IgA than wild-type animals. Secondary immunization of OVA-immunized IL-23p19−/− mice with TNP-OVA resulted in significantly reduced levels of serum TNP-specific IgG1 and IgG2a. Delayed-type hypersensitivity responses to methylated BSA in BSA-immunized IL-23p19−/− and IL-12p40−/− mice were inhibited compared with wild-type controls. Allotypic naive CD4+ T cells produced less IL-17 after stimulation in the presence of bacterial lipopeptides by IL-23p19−/− dendritic cells than by wild-type dendritic cells. Similarly, lymph node cells from IL-23p19−/− animals immunized in vivo and restimulated with the same Ag in vitro produced less IL-17 than controls. The authors conclude that IL-23 deficiency, manifested at the level of Ag specific memory CD4+ T cells, is a consequence of reduced production of IL-17.
Transplantation of human memory B cells
Allogeneic bone marrow transplantation (BMT) in humans renders the recipient susceptible to microbial infections for nearly a year following the procedure, but there are several reports of transfer of immunological memory. Lausen et al. (p. 3305 ) used clonal tracking to follow transplanted Ab-secreting cells (AbSC) that reacted specifically against the Hemophilus influenzae type b capsular polysaccharide (HibCP). A 10-mo-old girl with leukocyte adhesion deficiency type 1 received a BMT from her HLA-identical sister who had been immunized 7 days earlier with HibCP in tetanus toxoid. Immunization of the recipient 9 and 11 mo later with HibCP-tetanus toxoid resulted in high levels of HibCP Ab and AbSC. A total of 103 of 121 (85%) Ig VH gene sequences from HibCP-specific AbSC, isolated following the donor and each of the two recipient immunizations, contained a canonical sequence. Somatically acquired mutations in 79 of those 103 Ig VH gene sequences fell into 15 clusters, each sharing from 8 to 23 mutations. An additional cluster that shared 13–16 somatic mutations was found among 11 of the 18 noncanonical sequences. Therefore, 74% (90) of the 121 Ig VH gene sequences were assigned to 16 clonal progenies. B cells representing all 12 donor clones were retrieved from the recipient after each immunization (61% and 68%, respectively, of B cells). Little somatic diversification was seen in the memory B clones with repeated immunizations. The data demonstrate that memory B cells persist and respond for longer than 9 mo after BMT.
IgA and mucosal M cells
Secretory IgA (SIgA) responses in intestinal Peyer’s patch (PP) cells protect mucosa from infection with pathogens. SIgA is known to interact with specialized epithelial cells referred to as M (microfold) cells. However, the nature of the SIgA interaction with M cells and the fate of SIgA are unknown. Rey et al. (p. 3026 ) showed that IgA complexes containing green fluorescent protein covalently attached to the secretory component (green SIgA) associated with 17% of isolated PP cells; costaining with specific PE-labeled Abs identified them as dendritic (DC) and CD4+ T and B cells. Green SIgA injected into a ligated intestinal loop labeled 5% of the PP cells isolated from the site of injection; 3% and 2.5% of cells in two adjacent downstream PPs also were labeled. Immunofluorescence of PP sections showed that green SIgA colocalized with DC and CD4+ T cells in the subepithelial dome (SED) region. No colocalization was seen for B cells and green SIgA. Only DC, but not CD4+ T or B cells, incubated in vitro with FITC-labeled SIgA retained fluorescence after trypsin treatment. Laser scanning confocal microscopy on the SED region following loop injection of green SIgA showed that green SIgA was internalized only in DC in vivo. The authors conclude that IgA enters into the SED region of a PP via M cells and is internalized by DC to protect mucosal surfaces.
TLR2 induces Th2 polarization
The innate immune response to pathogens is mediated through Toll-like receptors (TLR), but the correlation of a specific TLR with a specific adaptive immune response has not been fully demonstrated. Redecke et al. (p. 2739 ) compared adaptive immune responses elicited when the TLR2 and TLR9 ligands, Pam3Cys and immunostimulatory DNA (ISS-ODN) respectively, were coinjected with OVA into C57BL/6 mice. ISS-ODN/OVA immunization resulted in high serum levels of IgG2a and production of IFN-γ from spleen cells stimulated with OVA in vitro. Pam3Cys/OVA immunization resulted in high serum levels of IgG1 and IgE, and an IL-13 response from stimulated splenocytes; these Pam3Cys/OVA responses were abrogated in TLR2−/− mice. CTL activity was more prominent in mice immunized with OVA in combination with ISS-ODN than with Pam3Cys. Intranasal OVA challenge of mice immunized with Pam3Cys/OVA, but not of mice immunized with ISS-ODN/OVA, had aggravated experimental asthma as measured by airway hypersensitivity, eosinophil numbers in the lung and Th2 cytokine release in bronchial lymph node cells in vitro. Naive bone marrow dendritic cells incubated with Pam3Cys/OVA had higher surface expression of the costimulatory molecule B7RP-1 than cells incubated with either ISS-ODN/OVA or OVA. RNAs for Th1 cytokines were induced in the cells incubated with ISS-ODN, whereas Pam3Cys induced Th2-associated cytokines. The authors conclude that the type of TLR stimulation polarizes the adaptive immune response to Th1 or Th2 and may influence the incidence of Th-associated diseases such as asthma.
Evading complement lysis
Factor H (fH), the regulator of the alternative pathway (AP) amplification loop of C, is comprised of 20 short consensus repeat (SCR) domains of ∼20 aa each. Surface proteins from several types of β-hemolytic streptococci, including Group B (GBS), and from serotype 3 pneumococci bind fH to evade C attack and opsonophagocytosis. The specific fH sites to which binding occurs were determined by Jarva et al. (p. 3111 ). They focused on the pneumococcal fH-binding inhibitor of complement (Hic) protein and the highly homologous GBS β protein. A recombinant construct consisting of fH SCR8–20 exhibited strongest binding to GBS β protein immobilized on a microchip as measured by surface plasmon resonance; SCR8–11 bound more strongly than SCR11–15 and SCR12–14, and SCR1–7 and SCR15–20 did not bind. Hic bound SCR8–11 strongly and SCR12–14 weakly. Mutant strains of GBS with deletions within β were used to map the binding region within aa 435–788. Interaction of heparin with its binding site at SCR13 inhibited the binding of SCR11–15 and slightly reduced the binding of SCR8–11 to immobilized β but had minimal effect on their binding to Hic. Peptide mapping studies revealed five putative binding sites for fH on β and three sites on Hic; all were rich in charged amino acids. The analyses show that homologous proteins from two different bacterial species bind to the same regions on fH to evade complement destruction.
Summaries written by Dorothy L. Buchhagen, Ph.D.