Pneumococcal capsular polysaccharide vaccine-mediated protection against serotype 3 Streptococcus pneumoniae in immunodeficient mice

The IgM response to pneumococcal polysaccharide vaccine is sufficient for conferring immunity

In mice, pneumococcal polysaccharide (PPS) vaccines generate antigen-specific IgM, IgG1, IgG2 and IgG3. Antibody and complement-dependent opsonophagocytosis correlates with the protection induced by PPS vaccines in vivo. Since IgM is a very efficient immunoglobulin isotype in activating the complement system, we evaluated whether anti-PPS IgM alone is sufficient to confer protective immunity to Streptococcus pneumoniae. We found that immunization of wildtype and activation-induced cytidine deaminase-deficient mice capable of producing only IgM with Pneumovax®23 generated comparable anti-PPS IgM and resistance to lethal systemic challenge with S. pneumoniae. These data suggests that an IgM response to PPS vaccines is sufficient for conferring immunity.

Isolation and Characterization of Human Monoclonal Antibodies to Pneumococcal Capsular Polysaccharide 3

The current pneumococcal capsular polysaccharide (PPS) conjugate vaccine (PCV13) is less effective against Streptococcus pneumoniae serotype 3 (ST3), which remains a major cause of pneumococcal disease and mortality. Therefore, dissecting structure-function relationships of human ST3 pneumococcal capsular polysaccharide (PPS3) antibodies may reveal characteristics of protective antibodies. Using flow cytometry, we isolated PPS3-binding memory B cells from pneumococcal vaccine recipients and generated seven PPS3-specific human monoclonal antibodies (humAbs). Five humAbs displayed ST3 opsonophagocytic activity, four induced ST3 agglutination in vitro, and four mediated both activities. Two humAbs, namely, C10 and C27, that used the same variable heavy (VH) and light (VL) chain domains (VH3-9*01/VL2-14*03) both altered ST3 gene expression in vitro; however, C10 had fewer VL somatic mutations, higher PPS3 affinity, and promoted in vitro ST3 opsonophagocytic and agglutinating activity, whereas C27 did not. In C57BL/6 mice, both humAbs reduced nasopharyngeal colonization with ST3 A66 and a clinical strain, B2, and prolonged survival following lethal A66 intraperitoneal infection, but only C10 protected against lethal intranasal infection with the clinical strain. After performing VL swaps, C10VH/C27VL exhibited reduced ST3 binding and agglutination, but C27VH/C10VL binding was unchanged. However, both humAbs lost the ability to reduce colonization in vivo when their light chains were replaced. Our findings associate the ability of PPS3-specific humAbs to reduce colonization with ST3 agglutination and opsonophagocytic activity, and reveal an unexpected role for the VL in their functional activity in vitro and in vivo. These findings also provide insights that may inform antibody-based therapy and identification of surrogates of vaccine efficacy against ST3. IMPORTANCE Despite the global success of vaccination with pneumococcal conjugate vaccines, serotype 3 (ST3) pneumococcus remains a leading cause of morbidity and mortality. In comparison to other vaccine-included serotypes, the ST3 pneumococcal capsular polysaccharide (PPS3) induces a weaker opsonophagocytic response, which is considered a correlate of vaccine efficacy. Previous studies of mouse PPS3 monoclonal antibodies identified ST3 agglutination as a correlate of reduced ST3 nasopharyngeal colonization in mice; however, neither the agglutinating ability of human vaccine-elicited PPS3 antibodies nor their ability to prevent experimental murine nasopharyngeal colonization has been studied. We generated and analyzed the functional and in vivo efficacy of human vaccine-elicited PPS3 monoclonal antibodies and found that ST3 agglutination associated with antibody affinity, protection in vivo, and limited somatic mutations in the light chain variable region. These findings provide new insights that may inform the development of antibody-based therapies and next-generation vaccines for ST3.

A Capsular Polysaccharide-Specific Antibody Alters Streptococcus pneumoniae Gene Expression during Nasopharyngeal Colonization of Mice

Pneumococcal conjugate vaccines (PCV) elicit opsonophagocytic (opsonic) antibodies to pneumococcal capsular polysaccharides (PPS) and reduce nasopharyngeal (NP) colonization by vaccine-included Streptococcus pneumoniae serotypes. However, nonopsonic antibodies may also be important for protection against pneumococcal disease. For example, 1E2, a mouse IgG1 monoclonal antibody (MAb) to the serotype 3 (ST3) PPS (PPS3), reduced ST3 NP colonization in mice and altered ST3 gene expression in vitro Here, we determined whether 1E2 affects ST3 gene expression in vivo during colonization of mice by performing RNA sequencing on NP lavage fluid from ST3-infected mice treated with 1E2, a control MAb, or phosphate-buffered saline. Compared to the results for the controls, 1E2 significantly altered the expression of over 50 genes. It increased the expression of the piuBCDA operon, which encodes an iron uptake system, and decreased the expression of dpr, which encodes a protein critical for resistance to oxidative stress. 1E2-mediated effects on ST3 in vivo required divalent binding, as Fab fragments did not reduce NP colonization or alter ST3 gene expression. In vitro, 1E2 induced dose-dependent ST3 growth arrest and altered piuB and dpr expression, whereas an opsonic PPS3 MAb, 5F6, did not. 1E2-treated bacteria were more sensitive to hydrogen peroxide and the iron-requiring antibiotic streptonigrin, suggesting that 1E2 may increase iron import and enhance sensitivity to oxidative stress. Finally, 1E2 also induced rapid capsule shedding in vitro, suggesting that this may initiate 1E2-induced changes in sensitivity to oxidative stress and gene expression. Our data reveal a novel mechanism of direct, antibody-mediated antibacterial activity that could inform new directions in antipneumococcal therapy and vaccine development.

Comparative evaluation of a newly developed 13-valent pneumococcal conjugate vaccine in a mouse model

Animal models facilitate evaluation of vaccine efficacy at relatively low cost. This study was a comparative evaluation of the immunogenicity and protective efficacy of a new 13-valent pneumococcal conjugate vaccine (PCV13) with a control vaccine in a mouse model.

After vaccination, anti-capsular antibody levels were evaluated by pneumococcal polysaccharide (PnP) enzyme-linked immunosorbent assay (ELISA) and opsonophagocytic killing assay (OPA). Also, mice were challenged intraperitoneally with 100-fold of the 50% lethal dose of Streptococcus pneumoniae.

The anti-capsular IgG levels against serotypes 1, 4, 7F, 14, 18C, 19A, and 19F were high (quartile 2 >1,600), while those against the other serotypes were low (Q2 ≤ 800). Also, the OPA titres were similar to those determined by PnP ELISA. Comparative analysis between new PCV13 and control vaccination group in a mouse model exhibited significant differences in serological immunity of a few serotypes and the range of anti-capsular IgG in the population. Challenge of wild-type or neutropenic mice with serotypes 3, 5, 6A, 6B, and 9V showed protective immunity despite of induced relatively low levels of anti-capsular antibodies. With comparison analysis, a mouse model should be adequate for evaluating serological efficacy and difference in the population level as preclinical trial.

Reduction of Streptococcus pneumoniae Colonization and Dissemination by a Nonopsonic Capsular Polysaccharide Antibody

Streptococcus pneumoniae colonization of the nasopharynx (NP) is a prerequisite for invasive pneumococcal disease (IPD). The marked reduction in IPD that followed the routine use of pneumococcal polysaccharide conjugate vaccines (PCVs) has been linked to reduced NP colonization with vaccine-included serotypes (STs), with the caveat that PCVs are less effective against pneumonia than against IPD. Although PCV-elicited opsonic antibodies that enhance phagocytic killing of the homologous ST are considered a key correlate of PCV-mediated protection, recent studies question this relationship for some STs, including ST3. Studies with monoclonal antibodies (MAbs) to the pneumococcal capsular polysaccharide (PPS) of ST3 (PPS3) have shown that nonopsonic, as well as opsonic, antibodies can each protect mice against pneumonia and sepsis, but the effect of these types of MAbs on NP colonization is unknown. In this study, we determined the effects of protective opsonic and nonopsonic PPS3 MAbs on ST3 NP colonization in mice. Our results show that a nonopsonic MAb reduced early NP colonization and prevented ST3 dissemination to the lungs and blood, but an opsonic MAb did not. Moreover, the opsonic MAb induced a proinflammatory NP cytokine response, but the nonopsonic MAb had an antiinflammatory effect. The effect of the nonopsonic MAb on colonization did not require its Fc region, but its antiinflammatory effect did. Our findings challenge the paradigm that opsonic MAbs are required to prevent NP colonization and suggest that further studies of the activity of nonopsonic antibodies could advance our understanding of mechanisms of PCV efficacy and provide novel correlates of protection.

Pneumococcal conjugate vaccines (PCVs) have markedly reduced the incidence of invasive pneumococcal disease (IPD). Vaccine-elicited pneumococcal polysaccharide (PPS) antibodies that enhance in vitro phagocyte killing of vaccine-included serotypes (STs) (opsonic antibodies) have been considered correlates of vaccine protection and are thought to exert their effect at the initial site of infection, the nasopharynx (NP). However, the data presented here show that this is not the necessarily the case. A nonopsonic PPS monoclonal antibody (MAb) reduced pneumococcal colonization and dissemination of its homologous ST in mice, but surprisingly, an opsonic PPS MAb to the same ST did not. These results reveal that PPS antibodies can work in different ways than previously thought, challenge the paradigm that opsonic antibodies are required to prevent IPD, and provide new insights into PCV efficacy that could lead to novel correlates of vaccine protection.

Beyond passive immunization: toward a nanoparticle-based IL-17 vaccine as first in class of future immune treatments

Nanoparticles occur naturally as part of repetitive molecular structures forming virus-like particles (VLPs). VLPs are powerful immune activators. Specifically, VLP can elicit a direct activation of B lymphocytes to trigger production of antibodies targeted at molecules chemically linked to the VLP. We here review recent data from genetics research, large-scale genomic sequencing, as well as clinical trials which suggest that a VLP-based vaccine against the signaling molecule IL-17 will be safe and effective in the common skin disease psoriasis, as well as other conditions. Active vaccination against IL-17 is capable of replacing the costly manufacture of antibodies currently in clinical use with huge implications for treatment availability and health economics.

Tailoring the Immune Response via Customization of Pathogen Gene Expression

The majority of studies focused on the construction and reengineering of bacterial pathogens have mainly relied on the knocking out of virulence factors or deletion/mutation of amino acid residues to then observe the microbe's phenotype and the resulting effect on the host immune response. These knockout bacterial strains have also been proposed as vaccines to combat bacterial disease. Theoretically, knockout strains would be unable to cause disease since their virulence factors have been removed, yet they could induce a protective memory response. While knockout strains have been valuable tools to discern the role of virulence factors in host immunity and bacterial pathogenesis, they have been unable to yield clinically relevant vaccines. The advent of synthetic biology and enhanced user-directed gene customization has altered this binary process of knockout, followed by observation. Recent studies have shown that a researcher can now tailor and customize a given microbe's gene expression to produce a desired immune response. In this commentary, we highlight these studies as a new avenue for controlling the inflammatory response as well as vaccine development.

Current concepts in host-microbe interaction leading to pneumococcal pneumonia

PURPOSE OF REVIEW

Infection with Streptococcus pneumoniae (pneumococcus) results in colonization, which can lead to local or invasive disease, of which pneumonia is the most common manifestation. Despite the availability of pneumococcal vaccines, pneumococcal pneumonia is the leading cause of community and inhospital pneumonia in the United States and globally. This article discusses new insights into the pathogenesis of pneumococcal disease.

RECENT FINDINGS

The host-microbe interactions that determine whether pneumococcal colonization will result in clearance or invasive disease are highly complex. This article focuses on new information in three areas that bear on the pathogenesis of pneumococcal disease: factors that govern colonization, the prelude to invasive disease, including effects on colonization and invasion of capsular serotype, pneumolysin, surface proteins that regulate complement deposition, biofilm formation and agglutination; the effect of coinfection with other bacteria and viruses on pneumococcal growth and virulence, including the synergistic effect of influenza virus; and the contribution of the host inflammatory response to the pathogenesis of pneumococcal pneumonia, including the effects of pattern recognition molecules, cells that enhance and modulate inflammation, and therapies that modulate inflammation, such as statins.

SUMMARY

Recent research on pneumococcal pathogenesis reveals new mechanisms by which microbial factors govern the ability of pneumococcus to progress from the state of colonization to disease and host inflammatory responses contribute to the development of pneumonia. These mechanisms suggest that therapies which modulate the inflammatory response could hold promise for ameliorating damage stemming from the host inflammatory response in pneumococcal disease.

Tissue-expressed B7x affects the immune response to and outcome of lethal pulmonary infection

B7x (B7-H4 or B7S1), a member of the B7 family, inhibits in vitro T cell proliferation and cytokine production by binding to an unidentified receptor on activated T cells, but its in vivo function remains largely unclear. We show that B7x protein was expressed in epithelial cells of the lung, but not in lymphoid tissues. To investigate the role of B7x in the lung, we determined the susceptibility of B7x-deficient (B7x(-/-)) mice to a lethal pulmonary infection with Streptococcus pneumoniae. B7x(-/-), but not B7-H3-deficient, mice were significantly more resistant to S. pneumoniae pulmonary infection than their wild-type (Wt) counterparts. B7x(-/-) mice had significantly lower bacterial burdens and levels of inflammatory cytokines in lungs as early as 12 h postinfection. They also had milder immunopathology that was localized in alveolar spaces, whereas Wt mice had severe inflammation that was perivascular. Control of infection in B7x(-/-) mice was associated with a marked increase in activated CD4 and CD8 T cells and fewer neutrophils in lungs, whereas the susceptible Wt mice had the opposite cellular profile. In B7x(-/-)Rag1(-/-) mice that lack T cells, reduction in bacterial burden was no longer observed. Control of S. pneumoniae and the increased survival observed was specific to the lung, because systemically infected B7x(-/-) mice were not resistant to infection. These data indicate that lung-expressed B7x negatively regulates T cells, and that in its absence, in B7x(-/-) mice, an enhanced T cell response contributed to reduced lethality in a pulmonary infection model with S. pneumoniae.

CD8+ T cells and risk for bacterial pneumonia and all-cause mortality among HIV-infected women

BACKGROUND

Bacterial pneumonia risk is disproportionately high among those infected with HIV. This risk is present across all CD4(+) T-cell levels (TCLs), suggesting that additional factors govern susceptibility. This study examines CD8(+) TCLs and risk for HIV-associated bacterial pneumonia and all-cause mortality.

METHODS

Demographic, clinical, and laboratory data were obtained for 885 HIV-infected women enrolled in the HIV Epidemiologic Research Study (HERS). Bacterial pneumonia cases were identified using clinical, microbiological, and radiographic criteria. CD8(+) TCLs were assessed at 6-month intervals. Statistical methods included Cox proportional hazards regression modeling and covariate-adjusted survival estimates.

RESULTS

Relative to a referent CD8(+) TCL of 401-800 cells per cubic millimeter, risk for bacterial pneumonia was significantly higher when CD8(+) TCLs were <400 (hazard ratio 1.65, P = 0.017, 95% confidence interval 1.10 to 2.49), after adjusting for age, CD4(+) TCL, viral load, and antiretroviral use. There was also a significantly higher risk of death when CD8(+) TCLs were ≤400 cells per cubic millimeter (hazard ratio 1.45, P = 0.04, 95% confidence interval 1.02 to 2.06). Covariate-adjusted survival estimates revealed shorter time to pneumonia and death in this CD8(+) TCL category, and the overall associations of the categorized CD8(+) TCL with bacterial pneumonia and all-cause mortality were each statistically significant (P = 0.017 and P < 0.0001, respectively).

CONCLUSIONS

CD8(+) TCL ≤400 cells per cubic millimeter was associated with increased risk for pneumonia and all-cause mortality in HIV-infected women in the HERS cohort, suggesting that CD8(+) TCL could serve as an adjunctive biomarker of pneumonia risk and mortality in HIV-infected individuals.

Controlled inflammatory responses in the lungs are associated with protection elicited by a pneumococcal surface protein A-based vaccine against a lethal respiratory challenge with Streptococcus pneumoniae in mice

Streptococcus pneumoniae is a pathogen of great importance worldwide. We have previously described the efficacy of a nasal vaccine composed of the pneumococcal surface protein A and the whole-cell pertussis vaccine as an adjuvant against a pneumococcal invasive challenge in mice. Spread of bacteria to the bloodstream was probably prevented by the high levels of systemic antibodies induced by the vaccine, but bacteria were only cleared from the lungs 3 weeks later, indicating that local immune responses may contribute to survival. Here we show that a strict control of inflammatory responses in lungs of vaccinated mice occurs even in the presence of high numbers of pneumococci. This response was characterized by a sharp peak of neutrophils and lymphocytes with a simultaneous decrease in macrophages in the respiratory mucosa at 12 h postchallenge. Secretion of interleukin-6 (IL-6) and gamma interferon (IFN-γ) was reduced at 24 h postchallenge, and the induction of tumor necrosis factor alpha (TNF-α) secretion, observed in the first hours postchallenge, was completely abolished at 24 h. Before challenge and at 12 h postchallenge, vaccinated mice displayed higher numbers of CD4(+) T, CD8(+) T, and B lymphocytes in the lungs. However, protection still occurs in the absence of each of these cells during the challenge, indicating that other effectors may be related to the prevention of lung injuries in this model. High levels of mucosal anti-PspA antibodies were maintained in vaccinated mice during the challenge, suggesting an important role in protection.

A serotype 3 pneumococcal capsular polysaccharide-specific monoclonal antibody requires Fcγ receptor III and macrophages to mediate protection against pneumococcal pneumonia in mice

Antibodies to pneumococcal capsular polysaccharide (PPS) are required for PPS-based vaccine-mediated protection against Streptococcus pneumoniae. Previous work established that 1E2, a mouse IgG1 to PPS3 that does not induce serotype 3 (ST3) S. pneumoniae killing by phagocytes in vitro, protects mice from death after intranasal infection with ST3, but its efficacy was abrogated in FcγR (F common gamma receptor)-deficient mice. In this study, we determined whether 1E2 efficacy against pulmonary ST3 infection requires FcγRIII. 1E2 did not protect FcγRIII-deficient (FcγRIII(-/-)) mice. Studies of the mechanism of 1E2-mediated effects showed that it resulted in a marked reduction in lung inflammation in ST3-infected wild-type (Wt [C57BL/6]) mice that was abrogated in FcγRIII(-/-) mice. 1E2 had no effect on early bacterial clearance in the lungs of ST3-infected Wt, FcγRIIB(-/-), or FcγRIII(-/-) mice, but it reduced levels of bacteremia and serum macrophage inflammatory protein-2) (MIP-2), interleukin-6 (IL-6), and tumor necrosis factor alpha (TNF-α) in Wt and FcγRIIB(-/-) mice, strains in which it is protective. As previous work showed that neutrophils were dispensable for 1E2 efficacy, we investigated whether macrophages are required for 1E2 efficacy against intranasal infection with ST3 and found that its efficacy was abrogated in Wt mice depleted of macrophages intranasally. In vitro studies revealed that1E2 promoted ST3 internalization by naïve alveolar macrophages but did not induce early intracellular killing. Macrophages from 1E2-treated ST3-infected mice studied ex vivo exhibited more apoptosis than those from FcγRIII(-/-) mice. These findings suggest that 1E2 mediates protection against ST3 in mice by affecting the inflammatory response, perhaps in part via macrophage apoptosis, rather than by inducing early bacterial clearance.

Mucosal immunization with an unadjuvanted vaccine that targets Streptococcus pneumoniae PspA to human Fcγ receptor type I protects against pneumococcal infection through complement- and lactoferrin-mediated bactericidal activity

Targeting an antigen to Fc receptors (FcR) can enhance the immune response to the antigen in the absence of adjuvant. Furthermore, we recently demonstrated that intranasal immunization with an FcγR-targeted antigen enhances protection against a category A intracellular mucosal pathogen, Francisella tularensis. To determine if a similar strategy could be applied to the important pathogen Streptococcus pneumoniae, we used an improved mucosal FcR-targeting strategy that specifically targets human FcγR type I (hFcγRI). A humanized single-chain antibody component in which the variable domain binds to hFcγRI [anti-hFcγRI (H22)] was linked in a fusion protein with the pneumococcal surface protein A (PspA). PspA is known to elicit protection against pneumococcal sepsis, carriage, and pneumonia in mouse models when administered with adjuvants. Anti-hFcγRI-PspA or recombinant PspA (rPspA) alone was used to intranasally immunize wild-type (WT) and hFcγRI transgenic (Tg) mice in the absence of adjuvant. The hFcγRI Tg mice receiving anti-hFcγRI-PspA exhibited elevated S. pneumoniae-specific IgA, IgG2c, and IgG1 antibodies in serum and bronchoalveolar lavage fluid. Neither immunogen was effective in protecting WT mice in the absence of adjuvant, but when PspA was targeted to hFcγRI as the anti-hFcγRI-PspA fusion, enhanced protection against lethal S. pneumoniae challenge was observed in the hFcγRI Tg mice compared to mice given nontargeted rPspA alone. Immune sera from the anti-hFcγRI-PspA-immunized Tg mice showed enhanced complement C3 deposition on bacterial surfaces, and protection was dependent upon an active complement system. Immune serum also showed an enhanced bactericidal activity directed against S. pneumoniae that appears to be lactoferrin mediated.

The innate immune response to Streptococcus pneumoniae in the lung depends on serotype and host response

Bacteremic pneumonia with some pneumococcal capsular serotypes, including serotype 3 (ST3), has been associated with a higher risk of death, whereas others, such as ST8, are associated with a lower risk. To provide a molecular basis for understanding such differences, we used oligo cDNA microarrays to analyze and compare the gene expression profiles of the lungs of Balb/c mice infected intranasally with either ST3, strain A66.1, or ST8, strain ATCC 6308 (6308). Compared to uninfected controls, infection with either A66.1 or 6308 led to inoculum-dependent expression of IFN-γ inducible CXC chemokines among other pro-inflammatory genes. To investigate the role that IFN-γ inducible chemokines CXCL9, CXCL10 and CXCL11 play in A66.1- and 6308-induced pneumonia, we examined the effect of the absence of their common receptor, CXCR3, on intranasal infection in CXCR3(-/-) (Balb/c) mice. Compared to wild type (WT) mice, virulence of A66.1 but not 6308 was attenuated in CXCR3(-/-) mice. A66.1-infected CXCR3(-/-) mice had fewer lung neutrophils and more alveolar macrophages 48 h after infection and fewer blood CFU 72 h after infection. Histopathological examination of lung sections revealed less inflammation among A66.1-infected CXCR3(-/-) than WT mice. The reduced virulence of A66.1 in CXCR3(-/-) mice suggests that inhibition of the functional activity of IFN-γ inducible chemokines modulates the host response to A66.1, in turn suggesting a novel approach to improve vaccine-mediated protection against ST3 pneumonia.

Designed reduction of Streptococcus pneumoniae pathogenicity via synthetic changes in virulence factor codon-pair bias

In this study, we used a previously described method of controlling gene expression with computer-based gene design and de novo DNA synthesis to attenuate the virulence of Streptococcus pneumoniae. We produced 2 S. pneumoniae serotype 3 (SP3) strains in which the pneumolysin gene (ply) was recoded with underrepresented codon pairs while retaining its amino acid sequence and determined their ply expression and pneumolysin production in vitro and their virulence in a mouse pulmonary infection model. Expression of ply and production of pneumolysin of the recoded SP3 strains were decreased, and the recoded SP3 strains were less virulent in mice than the wild-type SP3 strain or a Δply SP3 strain. Further studies showed that the least virulent recoded strain induced a markedly reduced inflammatory response in the lungs compared with the wild-type or Δply strain. These findings suggest that reducing pneumococcal virulence gene expression by altering codon-pair bias could hold promise for rational design of live-attenuated pneumococcal vaccines.

CD8+ cells enhance resistance to pulmonary serotype 3 Streptococcus pneumoniae infection in mice

Despite the success of the pneumococcal conjugate vaccine, pneumococcal pneumonia remains a significant clinical problem, and there is still much to learn about natural resistance and cellular immunity to pneumococcus. We investigated the role of T lymphocytes in resistance to serotype (ST) 3 Streptococcus pneumoniae in an intranasal infection model in C57BL/6 (wild-type [Wt]) and CD8(+) (CD8(-/-))- and CD4(+) (MHC class II(-/-))-deficient mice. CD8(-/-) mice exhibited significantly more bacterial dissemination and lung inflammation and a significantly more lethal phenotype than Wt mice. However, there was no difference in the bacterial dissemination, lung inflammation, or survival of Wt and MHC class II(-/-) mice. Perforin (Pfn)(-/-) and IFN-γ(-/-) mice, which were used to dissect the role of CD8(+) T cells in our model, also exhibited a more lethal survival phenotype than Wt mice. Comparison of lung chemokine/cytokine levels by Luminex and cellular recruitment by FACS in Wt mice and knockout strains revealed that CD8(-/-) and IFN-γ(-/-) mice, which had the most lethal survival phenotype, had more CD4(+)IL-17(+) T (Th17) cells, IL-17, neutrophil chemoattractants, and lung neutrophils, and fewer regulatory T cells than Wt mice. CD4(+) T cell depletion improved the survival of ST-infected CD8(-/-) mice, and survival studies in Th17-deficient mice revealed that the Th17 response was dispensable for ST3 resistance in our model. Taken together, these findings demonstrate that CD8(+) cells are required, but CD4(+) T cells are dispensable for resistance to ST3 pneumonia in mice and suggest a previously unsuspected role for CD8(+) cells in modulating the inflammatory response to ST3.

Characterization of gene use and efficacy of mouse monoclonal antibodies to Streptococcus pneumoniae serotype 8

Streptococcus pneumoniae is the most common cause of community-acquired pneumonia in the United States and globally. Despite the availability of pneumococcal capsular polysaccharide (PPS) and protein conjugate-based vaccines, the prevalence of antibiotic-resistant pneumococcal strains, serotype (ST) replacement in nonconjugate vaccine strains, and uncertainty as to whether the PPS vaccine that is used in adults protects against pneumonia emphasize the need for continued efforts to understand the nature of protective PPS antibody responses. In this study, we generated mouse monoclonal antibodies (MAbs) to a conjugate consisting of the PPS of serotype 8 (PPS8) S. pneumoniae and tetanus toxoid. Thirteen MAbs, including four IgMs that bound to PPS8 and phosphorylcholine (PC) and five IgMs and four IgG1s that bound to PPS8 but not PC, were produced, and their nucleotide sequences, epitope and fine specificity, and efficacy against lethal challenge with ST8 S. pneumoniae were determined. MAbs that bound to PPS8 exhibited gene use that was distinct from that exhibited by MAbs that bound to PC. Only PPS8-binding MAbs that did not bind PC were protective in mice. All 13 MAbs used germ line variable-region heavy (V(H)) and light (V(L)) chain genes, with no evidence of somatic hypermutation. Our data reveal a relationship between PPS specificity and V(H) gene use and MAb efficacy in mice. These findings provide insight into the relationship between antibody molecular structure and function and hold promise for the development of novel surrogates for pneumococcal vaccine efficacy.

The efficacy of pneumococcal capsular polysaccharide-specific antibodies to serotype 3 Streptococcus pneumoniae requires macrophages

The efficacy of antibody immunity against Streptococcus pneumoniae stems from the ability of opsonic, serotype (ST)-specific antibodies to pneumococcal capsular polysaccharide (PPS) to facilitate killing of the homologous ST by host phagocytes. However, PPS-specific antibodies have been identified that are protective in mice, but do not promote opsonic killing in vitro, raising the question of how they mediate protection in vivo. To probe this question, we investigated the dependence of antibody efficacy against lethal systemic (intraperitoneal, i.p.) infection with Streptococcus pneumoniae serotype 3 (ST3) on macrophages and neutrophils for the following PPS3-specific monoclonal antibodies (MAbs) in survival experiments in mice using a non-opsonic human IgM (A7), a non-opsonic mouse IgG1 (1E2) and an opsonic mouse IgG1 (5F6). The survival of A7- and PPS3-specific and isotype control MAb-treated neutrophil-depleted and neutrophil-sufficient and macrophage-depleted and macrophage-sufficient mice were determined after i.p. challenge with ST3 strains 6303 and WU2. Neutrophils were dispensable for A7 and the mouse MAbs to mediate protection in this model, but macrophages were required for the efficacy of A7 and optimal mouse MAb-mediated protection. For A7-treated mice, macrophage-depleted mice had higher blood CFU, cytokines and peripheral neutrophil levels than macrophage-sufficient mice, and macrophage-sufficient mice had lower tissue bacterial burdens than control MAb-treated mice. These findings demonstrate that macrophages contribute to opsonic and non-opsonic PPS3-specific MAb-mediated protection against ST3 infection by enhancing bacterial clearance and suggest that neutrophils do not compensate for the absence of macrophages in the model used in this study.

Generation of antibody responses to pneumococcal capsular polysaccharides is independent of CD1 expression in mice

Streptococcus pneumoniae is a bacterial microorganism that frequently causes serious infection, particularly in children and the elderly. Protection against infection with S. pneumoniae is based mainly on the generation of antibodies to the pneumococcal capsular polysaccharides (caps-PS), but the mechanisms responsible for the generation of anticapsular antibodies remain incompletely understood. The aim of the present study was to evaluate the role of CD1-restricted T cells in the antibody response to caps-PS. When immunized with Pneumo23, wild-type mice and CD1 knockout mice on BALB/c and C57BL/6 backgrounds generated immunoglobulin M (IgM) and IgG antibody responses to soluble caps-PS that were comparable. Similar results were obtained after immunization with heat-inactivated S. pneumoniae. The IgM and IgG antibody response of wild-type mice to Pneumo23 was not affected by an antagonizing monoclonal anti-CD1 antibody treatment. In summary, our data provide evidence that the antibody response to caps-PS is generated independently of CD1 expression.

Efficacy of opsonic and nonopsonic serotype 3 pneumococcal capsular polysaccharide-specific monoclonal antibodies against intranasal challenge with Streptococcus pneumoniae in mice

Serotype-specific antibodies to pneumococcal capsular polysaccharide (PPS) are a critical component of vaccine-mediated immunity to Streptococcus pneumoniae. In this study, we investigated the in vitro opsonophagocytic activities of three PPS-specific mouse immunoglobulin G1 monoclonal antibodies (MAbs), 1E2, 5F6, and 7A9, and determined their in vivo efficacies against intranasal challenge with WU2, a serotype 3 pneumococcal strain, in normal and immunodeficient mice. The MAbs had different in vitro activities in a pneumococcal killing assay: 7A9 enhanced killing by mouse neutrophils and J774 cells in the presence of a complement source, whereas 5F6 promoted killing in the absence, but not the presence, of complement, and 1E2 did not promote killing under any conditions. Nonetheless, all three MAbs protected normal and complement component 3-deficient mice from a lethal intranasal challenge with WU2 in passive-immunization experiments in which 10 mug of the MAbs were administered intraperitoneally before intranasal challenge. In contrast, only 1E2 protected Fcgamma receptor IIB knockout (FcgammaRIIB KO) mice and mice that were depleted of neutrophils with the MAb RB6, whereas 7A9 and 5F6 required neutrophils and FcgammaRIIB to mediate protection. Conversely, 7A9 and 5F6 protected FcgammaR KO mice, but 1E2 did not. Hence, the efficacy of 1E2 required an activating FcgammaR(s), whereas 5F6 and 7A9 required the inhibitory FcgammaR (FcgammaRIIB). Taken together, our data demonstrate that both MAbs that do and do not promote pneumococcal killing in vitro can mediate protection in vivo, although their efficacies depend on different host receptors and/or components.