Protective role of lung surfactant protein D in a murine model of invasive pulmonary aspergillosis

The role of surfactant protein D in the colonisation of the respiratory tract and onset of bacteraemia during pneumococcal pneumonia

We have shown previously that surfactant protein D (SP-D) binds and agglutinates Streptococcus pneumoniae in vitro. In this study, the role of SP-D in innate immunity against S. pneumoniae was investigated in vivo, by comparing the outcome of intranasal infection in surfactant protein D deficient (SP-D-/-) to wildtype mice (SP-D+/+). Deficiency of SP-D was associated with enhanced colonisation and infection of the upper and lower respiratory tract and earlier onset and longer persistence of bacteraemia. Recruitment of neutrophils to inflammatory sites in the lung was similar in both strains mice in the first 24 hrs post-infection, but different by 48 hrs. T cell influx was greatly enhanced in SP-D-/- mice as compared to SP-D+/+ mice. Our data provides evidence that SP-D has a significant role to play in the clearance of pneumococci during the early stages of infection in both pulmonary sites and blood.

Role and regulation of lung collectins in allergic airway sensitization

Inhalation of allergens in atopic patients results in a characteristic inflammatory response while in normal, healthy individuals it elicits no symptoms. The mechanisms by which the pulmonary immune system accomplishes elimination of inhaled particles and suppression of the ensuing inflammatory response are poorly understood. Based on their structural uniqueness, specific localization and functional versatility the hydrophilic surfactant proteins [surfactant protein (SP)-A and SP-D] are important candidate regulators of these processes. Recent studies in our laboratory and others indicated significant changes in levels of these molecules during the asthmatic response in animal models as well as in asthmatic patients. Because of their capability to directly inhibit T-cell activation and T-cell-dependent allergic inflammatory events, SP-A and SP-D may be significant contributors to the local control of T-helper (Th)2-type inflammation in the airways. This review will discuss their relevant structural-functional features and recent evidence supporting the hypothesis that SP-A and SP-D have a role in regulation of allergic airway sensitization.

Role of collectins in innate immunity against aspergillosis

The protective role of lung surfactant proteins SP-A, SP-D and MBL in the host defense against both allergic and invasive aspergillosis was identified and established by a series of in vitro and in vivo studies. Therapeutic administration of SP-D and MBL proteins in a murine model of pulmonary invasive aspergillosis rescued mice from death. In mice mimicking human allergic bronchopulmonary aspergillosis, SP-A and SP-D suppressed IgE levels, eosinophilia, pulmonary cellular infiltration and cause a marked shift from a pathogenic Th2 to a protective Th1 cytokine profile. SP-A and SP-D knock-out mice studies made significant contributions in understanding the mechanisms by which SP-A and SP-D modulate the host defense response in patients suffering from pulmonary allergies and infections. The results suggested that individuals with any structural or functional defects in these innate immune molecules due to genetic variations might be susceptible to aspergillosis. SNPs in SP-A2 and MBL genes showed significant associations with patients of allergic bronchopulmonary aspergillosis in an Indian population. The patients carrying either one or both of GCT and AGG alleles of SP-A2 and patients with A allele at position 1011 of MBL had markedly higher eosinophilia, total IgE antibodies and lower FEV1 (the clinical markers of ABPA). Our results show that collectins play an important role in Aspergillus mediated allergies and infections.

Surfactant proteins SP-A and SP-D: structure, function and receptors

Surfactant proteins, SP-A and SP-D, are collagen-containing C-type (calcium dependent) lectins called collectins, which contribute significantly to surfactant homeostasis and pulmonary immunity. These highly versatile innate immune molecules are involved in a range of immune functions including viral neutralization, clearance of bacteria, fungi and apoptotic and necrotic cells, down regulation of allergic reaction and resolution of inflammation. Their basic structures include a triple-helical collagen region and a C-terminal homotrimeric lectin or carbohydrate recognition domain (CRD). The trimeric CRDs can recognize carbohydrate or charge patterns on microbes, allergens and dying cells, while the collagen region can interact with receptor molecules present on a variety of immune cells in order to initiate clearance mechanisms. Studies involving gene knock-out mice, murine models of lung hypersensitivity and infection, and functional characterization of cell surface receptors have revealed the diverse roles of SP-A and SP-D in the control of lung inflammation. A recently proposed model based on studies with the calreticulin-CD91 complex as a receptor for SP-A and SP-D has suggested an anti-inflammatory role for SP-A and SP-D in naïve lungs which would help minimise the potential damage that continual low level exposure to pathogens, allergens and apoptosis can cause. However, when the lungs are overwhelmed with exogenous insults, SP-A and SP-D can assume pro-inflammatory roles in order to complement pulmonary innate and adaptive immunity. This review is an update on the structural and functional aspects of SP-A and SP-D, with emphasis on their roles in controlling pulmonary infection, allergy and inflammation. We also try to put in perspective the controversial subject of the candidate receptor molecules for SP-A and SP-D.

Surfactant protein D deficiency influences allergic immune responses

BACKGROUND

The collectin surfactant protein D (SP-D) confers protection against pulmonary infection and inflammation. Recent data suggest a role for SP-D in the modulation of allergic inflammation.

OBJECTIVE

The aim of this study is to characterize the immune responses of SP-D-deficient (SP-D(-/-)) mice in a kinetic model of allergic inflammation. We determined whether allergic parameters were enhanced in SP-D(-/-) mice in vivo. Further, we examined whether functional immune responses in vitro such as lymphocyte proliferation (LP) and cytokine production were modulated in the absence of SP-D.

METHODS

In vivo, wild-type (WT) and SP-D(-/-) mice were sensitized and challenged with the allergen ovalbumin (OVA) and assessed for allergic parameters (bronchoalveolar lavage (BAL) eosinophils, IL-13 production, pulmonary IFN-gamma, IL-10 expression) at early time points (1 and 3 days of challenge) in comparison with late time points (7 days of challenge). In vitro, spleen cells from WT and SP-D(-/-) mice were stimulated with the mitogen concanavalin A (ConA) and lipid A (LpA) and analysed for LP, IL-13 and IFN-gamma production. Toll-like receptor 4 (TLR4), ligand for LpA, was assessed by mRNA expression and immunohistochemistry in vivo.

RESULTS

Following allergen exposure in vivo, SP-D(-/-) mice expressed higher BAL eosinophils and IL-13 concentrations and lower IFN-gamma expression at early time points compared with WT mice. IL-10 expression was increased at early time points in SP-D(-/-) compared with WT mice. Allergen-induced TLR4 expression was increased in WT, but not in SP-D(-/-) mice. After stimulation with LpA and ConA in vitro LP was increased and IFN-gamma concentration was decreased in SP-D(-/-) mice.

CONCLUSION

SP-D may be critical for the modulation of early stages of allergic inflammation in vivo.

Anti-Aspergillus fumigatus efficacy of pentraxin 3 alone and in combination with antifungals

The collectin pentraxin 3 (PTX3) is an essential component of host resistance to pulmonary aspergillosis. Here we examined the protective effects of administration of PTX3 alone or together with deoxycholate amphotericin B (Fungizone) or liposomal amphotericin B (AmBisome) against invasive aspergillosis in a murine model of allogeneic bone marrow transplantation. PTX3, alone or in combination with the polyenes, was given intranasally or parenterally either before, in concomitance with, or after the intranasal infection with Aspergillus fumigatus conidia. Mice were monitored for resistance to infection and parameters of innate and adaptive T-helper immunity. The results showed the following: (i) complete resistance to infection and reinfection was observed in mice treated with PTX3 alone; (ii) the protective effect of PTX3 was similar or superior to that observed with liposomal amphotericin B or deoxycholate amphotericin B, respectively; (iii) protection was associated with accelerated recovery of lung phagocytic cells and T-helper-1 lymphocytes and concomitant decrease of inflammatory pathology; and (iv) PTX3 potentiated the therapeutic efficacy of suboptimal doses of either antimycotic drug. Together, these data suggest the potential therapeutic use of PTX3 either alone or as an adjunctive therapy in A. fumigatus infections.

Coccidioides posadasii infection alters the expression of pulmonary surfactant proteins (SP)-A and SP-D

BACKGROUND

Coccidioidomycosis or Valley Fever is caused by Coccidioides in Southwest US and Central America. Primary pulmonary infection is initiated by inhalation of air-borne arthroconidia. Since, lung is the first organ that encounters arthroconidia, different components of the pulmonary innate immune system may be involved in the regulation of host defense. Pulmonary surfactant proteins (SP)-A and SP-D have been recognized to play an important role in binding and phagocytosis of various microorganisms, but their roles in Coccidioides infection are not known.

METHODS

In this study, we studied the changes in amounts of pulmonary SP-A, SP-D and phospholipid in murine model of Coccidioides posadasii infection, and binding of SP-A and SP-D to Coccidioidal antigens. Mice were challenged intranasally with a lethal dose of C. posadasii (n = 30 arthroconidia) and bronchoalveolar lavage fluid (BALF) samples were collected on day 10, post infection. In another group of animals, mice were immunized with protective formalin killed spherule (FKS) vaccine prior to infection. The concentrations of BALF SP-A, SP-D, total phospholipid were measured using enzyme linked immunosorbent assay and biochemical assays.

RESULTS

We found that in lavage fluid samples of C. posadasii infected mice, the concentrations of total phospholipid, SP-A and SP-D were 17 % (SEM 3.5, p < 0.001), 38 % (SEM 5.8, p < 0.001) and 4 % (SEM 1.3, p < 0.001) of those in lavage fluid samples of non-infected control mice, respectively. However, the concentrations of SP-A and SP-D remained unchanged in BALF samples of C. posadasii protected mice after immunization with FKS vaccine. Also, we found that both SP-A and SP-D bind to Coccidiodal antigens.

CONCLUSION

Our results suggest that the C. posadasii infection perturbs the pulmonary SP-A, SP-D, and phospholipids, potentially enabling the disease progression and promoting fungal dissemination.

Surfactant protein D is present in the human female reproductive tract and inhibits Chlamydia trachomatis infection

Surfactant protein D (SP-D) is a lung collectin involved in innate host defence mechanisms in the lung. SP-D is also expressed at other mucosal sites throughout the human body. In the present study, we show that SP-D mRNA and protein are expressed in the human female reproductive tract. SP-D protein was localized in the apical portion of the reproductive epithelial cells. We also demonstrate that endometrial and endocervical cell lines and primary endocervical cells in culture produce SP-D mRNA and protein. Chlamydia trachomatis is an intracellular pathogen that infects the female reproductive tract, primarily the cervix, and is responsible for the most prevalent infectious disease in the USA. Untreated chlamydial infections of the female reproductive tract often result in sterility of the infected woman. Since SP-D protein is produced in cervical glands, we examined the effect of SP-D on chlamydial infection of cervical epithelial cells in vitro. We found that SP-D protein inhibits the infection of HeLa cells (an endocervical epithelial cell line) by C. trachomatis in a dose-dependent manner. We further demonstrate that the SP-D lectin-binding domain is involved in inhibiting infection of HeLa cells by Chlamydia. In conclusion, we detected SP-D in the female reproductive tract and determined that one of the functions of the SP-D protein may be to protect cervical epithelial cells from infection by C. trachomatis.

Pulmonary collectins modulate strain-specific influenza a virus infection and host responses

Collectins are secreted collagen-like lectins that bind, agglutinate, and neutralize influenza A virus (IAV) in vitro. Surfactant proteins A and D (SP-A and SP-D) are collectins expressed in the airway and alveolar epithelium and could have a role in the regulation of IAV infection in vivo. Previous studies have shown that binding of SP-D to IAV is dependent on the glycosylation of specific sites on the HA1 domain of hemagglutinin on the surface of IAV, while the binding of SP-A to the HA1 domain is dependent on the glycosylation of the carbohydrate recognition domain of SP-A. Here, using SP-A and SP-D gene-targeted mice on a common C57BL6 background, we report that viral replication and the host response as measured by weight loss, neutrophil influx into the lung, and local cytokine release are regulated by SP-D but not SP-A when the IAV is glycosylated at a specific site (N165) on the HA1 domain. SP-D does not protect against IAV infection with a strain lacking glycosylation at N165. With the exception of a small difference on day 2 after infection with X-79, we did not find any significant difference in viral load in SP-A(-/-) mice with either IAV strain, although small differences in the cytokine responses to IAV were detected in SP-A(-/-) mice. Mice deficient in both SP-A and SP-D responded to IAV similarly to mice deficient in SP-D alone. Since most strains of IAV currently circulating are glycosylated at N165, SP-D may play a role in protection from IAV infection.

Surfactant proteins A and D enhance the phagocytosis of Chlamydia into THP-1 cells

Chlamydiae are intracellular bacterial pathogens that infect mucosal surfaces, i.e., the epithelium of the lung, genital tract, and conjunctiva of the eye, as well as alveolar macrophages. In the present study, we show that pulmonary surfactant protein A (SP-A) and surfactant protein D (SP-D), lung collectins involved in innate host defense, enhance the phagocytosis of Chlamydia pneumoniae and Chlamydia trachomatis by THP-1 cells, a human monocyte/macrophage cell line. We also show that SP-A is able to aggregate both C. trachomatis and C. pneumoniae but that SP-D only aggregates C. pneumoniae. In addition, we found that after phagocytosis in the presence of SP-A, the number of viable C. trachomatis pathogens in the THP-1 cells 48 h later was increased approximately 3.5-fold. These findings suggest that SP-A and SP-D interact with chlamydial pathogens and enhance their phagocytosis into macrophages. In addition, the chlamydial pathogens internalized in the presence of collectins are able to grow and replicate in the THP-1 cells after phagocytosis.

Binding and agglutination of Streptococcus pneumoniae by human surfactant protein D (SP-D) vary between strains, but SP-D fails to enhance killing by neutrophils

Recombinant human surfactant protein D (SP-D) expressed in Escherichia coli, consisting of the head and neck regions of the native molecule, bound to all strains of Streptococcus pneumoniae that were tested, but the extent of binding varied between strains of differing capsular serotypes. The recombinant protein expressed in the yeast Pichia pastoris did not bind. Full-length native SP-D aggregated pneumococci in a calcium-dependent manner that was inhibited by maltose acting as a competitive sugar. The ability of SP-D to modulate the uptake and killing of pneumococci by human neutrophils was also addressed. Neither recombinant truncated SP-D nor native full-length SP-D enhanced the killing of pneumococci by human neutrophils. Aggregation of pneumococci varied not only between strains of the same multilocus sequence type and different serotypes but also between strains of the same serotype. However, use of recombinant strains in which the serotype had been changed showed that the degree of aggregation was influenced by the capsular type. Indeed, a 19F serotype strain which was not aggregated by SP-D did exhibit aggregation when the original isogenic strain was capsule switched to capsular serotype 3. However, although our results show that SP-D is capable of aggregating most pneumococci, no correlation between the degree of aggregation and the capsule or multilocus sequence type of the pneumococcus was clearly apparent. Therefore, although the capsule serotype is not the only determinant of aggregation by SP-D, the data presented here indicate that it does have a role to play.

High-resolution structural insights into ligand binding and immune cell recognition by human lung surfactant protein D

Lung surfactant protein D (SP-D) can directly interact with carbohydrate residues on pulmonary pathogens and allergens, stimulate immune cells, and manipulate cytokine and chemokine profiles during the immune response in the lungs. Therapeutic administration of rfhSP-D, a recombinant homotrimeric fragment of human SP-D comprising the alpha-helical coiled-coil neck plus three CRDs, protects mice against lung allergy and infection caused by the fungal pathogen Aspergillus fumigatus. The high resolution crystal structures of maltose-bound rfhSP-D to 1.4A, and of rfhSP-D to 1.6A, define the fine detail of the mode and nature of carbohydrate recognition and provide insights into how a small fragment of human SP-D can bind to allergens/antigens or whole pathogens, and at the same time recruit and engage effector cells and molecules of humoral immunity. A previously unreported calcium ion, located on the trimeric axis in a pore at the bottom of the funnel formed by the three CRDs and close to the neck-CRD interface, is coordinated by a triad of glutamate residues which are, to some extent, neutralised by their interactions with a triad of exposed lysine residues in the funnel. The spatial relationship between the neck and the CRDs is maintained internally by these lysine residues, and externally by a glutamine, which forms a pair of hydrogen-bonds within an external cleft at each neck-CRD interface. Structural links between the central pore and the cleft suggest a possible effector mechanism for immune cell surface receptor binding in the presence of bound, extended natural lipopolysaccharide and phospholipid ligands. The structural requirements for such an effector mechanism, involving both the trimeric framework for multivalent ligand binding and recognition sites formed from more than one subunit, are present in both native hSP-D and rfhSP-D, providing a possible explanation for the significant biological activity of rfhSP-D.

Surfactant proteins A and D inhibit the growth of Gram-negative bacteria by increasing membrane permeability

The pulmonary collectins, surfactant proteins A (SP-A) and D (SP-D), have been reported to bind lipopolysaccharide (LPS), opsonize microorganisms, and enhance the clearance of lung pathogens. In this study, we examined the effect of SP-A and SP-D on the growth and viability of Gram-negative bacteria. The pulmonary clearance of Escherichia coli K12 was reduced in SP-A-null mice and was increased in SP-D-overexpressing mice, compared with strain-matched wild-type controls. Purified SP-A and SP-D inhibited bacterial synthetic functions of several, but not all, strains of E. coli, Klebsiella pneumoniae, and Enterobacter aerogenes. In general, rough E. coli strains were more susceptible than smooth strains, and collectin-mediated growth inhibition was partially blocked by coincubation with rough LPS vesicles. Although both SP-A and SP-D agglutinated E. coli K12 in a calcium-dependent manner, microbial growth inhibition was independent of bacterial aggregation. At least part of the antimicrobial activity of SP-A and SP-D was localized to their C-terminal domains using truncated recombinant proteins. Incubation of E. coli K12 with SP-A or SP-D increased bacterial permeability. Deletion of the E. coli OmpA gene from a collectin-resistant smooth E. coli strain enhanced SP-A and SP-D-mediated growth inhibition. These data indicate that SP-A and SP-D are antimicrobial proteins that directly inhibit the proliferation of Gram-negative bacteria in a macrophage- and aggregation-independent manner by increasing the permeability of the microbial cell membrane.

Association of polymorphisms in the collagen region of human SP-A1 and SP-A2 genes with pulmonary tuberculosis in Indian population

Surfactant protein A (SP-A) binds to and modulates phagocytosis of Mycobacterium tuberculosis by macrophages. We investigated the relationship between polymorphisms in the collagen regions of SP-A1 and SP-A2 genes and pulmonary tuberculosis. In the present study, seven single nucleotide polymorphisms (SNPs) (4 exonic and 3 intronic) have been identified in the collagen regions of SP-A1 and SP-A2 genes in Indian population. Two intronic polymorphisms, SP-A1C1416T ((p = 0.0000, odds ratio (OR) = 20.767,95% CI: 8.315-OR<51.870) and SP-A2C1382G (p = 0.0054; OR = 3.675, 95% CI: 1.400< OR<9.644), showed significant association with pulmonary tuberculosis (number of patients = 10, number of controls = 7). A redundant SNPA1660G of SP-A2gene showed significant association with pulmonary tuberculosis (number of patients = 17, number of controls = 19, p = 0.0000, OR = 8.94,95% CI: 3.311<OR<24.126). This polymorphism, when existing along with a non-redundant polymorphism, SP-A2G1649C (Ala91Pro) resulted in a stronger association with pulmonary tuberculosis (number of patients = 17, number of controls = 19, p = 0.000, OR = 16.3, 95% CI: 7.8644<OR<33.9244). The results indicated that these SNPs in the collagen region of SP-A2 may be one of the contributing factors to the genetic predisposition to pulmonary tuberculosis.

Surfactant protein D binds selectively to Klebsiella pneumoniae lipopolysaccharides containing mannose-rich O-antigens

Surfactant protein D (SP-D) plays important roles in the regulation of innate immune responses in the lung. We have previously shown that SP-D can agglutinate and enhance the macrophage-dependent killing of specific unencapsulated phase variants of Klebsiella pneumoniae. In the present studies, we used 16 clinical isolates of Klebsiella representing four O-serotypes and examined the interaction of SP-D with their isolated LPSs. Although SP-D bound to the core oligosaccharide of rough LPS from all isolates, it selectively bound to smooth forms of LPS expressed by O-serotypes with mannose-rich repeating units in their O-polysaccharides. SP-D was more potent in agglutinating unencapsulated phase variants of O-serotypes expressing these SP-D "reactive" O-polysaccharides, and more effectively inhibited the adhesion of these serotypes to lung epithelial cells. This novel anti-adhesion activity required the multimerization of trimeric SP-D subunits (dodecamers). Klebsiella serotypes expressing "nonreactive" LPS O-Ags were isolated at a significantly higher frequency from patients with K. pneumoniae. Our findings suggest that SP-D plays important roles in the clearance of opportunistic Gram-negative bacteria and contributes to known serotypic differences in the pathogenicity of Klebsiella through specific interactions with O-polysaccharides.

Protection of killer antiidiotypic antibodies against early invasive aspergillosis in a murine model of allogeneic T-cell-depleted bone marrow transplantation

Antiidiotypic monoclonal antibodies (MAbs) representing the internal image of a yeast killer toxin (KT) have therapeutic potential against several fungal infections. The efficacy of KT MAbs against Aspergillus fumigatus was investigated in a mouse model of T-cell-depleted allogeneic bone marrow transplantation (BMT) with invasive pulmonary aspergillosis. Mice were highly susceptible to infection at 3 days post-BMT, when profound neutropenia was observed both in the periphery and in the lungs. Treatment with KT MAbs protected the mice from infection, as judged by the long-term survival and decreased pathology associated with inhibition of fungal growth and hyphal development in the lungs. In vitro, similar to polymorphonuclear neutrophils, KT MAbs significantly inhibited the hyphal development and metabolic activity of germinated Aspergillus conidia. These results indicate that mimicking the action of neutrophils could be a strategy through which KT MAbs exert therapeutic efficacy in A. fumigatus infections.