Annotation and genetic diversity of the chicken collagenous lectins

Goose surfactant protein A inhibits the growth of avian pathogenic Escherichia coli via an aggregation-dependent mechanism that decreases motility and increases membrane permeability

Pulmonary collectins have been reported to bind carbohydrates on pathogens and inhibit infection by agglutination, neutralization, and opsonization. In this study, surfactant protein A (SP-A) was identified from goose lung and characterized at expression- and agglutination-functional levels. The deduced amino acid sequence of goose surfactant protein A (gSP-A) has two characteristic structures: a shorter, collagen-like region and a carbohydrate recognition domain. The latter contains two conserved motifs in its Ca²⁺-binding site: EPN (Glu-Pro-Asn) and WND (Trp-Asn-Asp). Expression analysis using qRT-PCR and fluorescence IHC revealed that gSP-A was highly expressed in the air sac and present in several other tissues, including the lung and trachea. We went on to produce recombinant gSP-A (RgSP-A) using a baculovirus/insect cell system and purified using a Ni²⁺ affinity column. A biological activity assay showed that all bacterial strains tested in this study were aggregated by RgSP-A, but only Escherichia coli AE17 (E. coli AE17, O2) and E. coli AE158 (O78) were susceptible to RgSP-A-mediated growth inhibition at 2-6 h. Moreover, the swarming motility of the two bacterial strains were weakened with increasing RgSP-A concentration, and their membrane permeability was compromised at 3 h, as determined by flow cytometry and laser confocal microscopy. Therefore, RgSP-A is capable of reducing bacterial viability of E. coli O2 and O78 via an aggregation-dependent mechanism which involves decreasing motility and increasing the bacterial membrane permeability. These data will facilitate detailed studies into the role of gSP-A in innate immune defense as well as for development of antibacterial agents.

Avian surfactant protein (SP)-A2 first arose in an early tetrapod before the divergence of amphibians and gradually lost the collagen domain

The air-liquid interface of the mammalian lung is lined with pulmonary surfactants, a mixture of specific proteins and lipids that serve a dual purpose-enabling air-breathing and protection against pathogens. In mammals, surfactant proteins A (SP-A) and D (SP -D) are involved in innate defence of the lung. Birds seem to lack the SP-D gene, but possess SP-A2, an additional SP-A-like gene. Here we investigated the evolution of the SP-A and SP-D genes using computational gene prediction, homology, simulation modelling and phylogeny with published avian and other vertebrate genomes. PCR was used to confirm the identity and expression of SP-A analogues in various tissue homogenates of zebra finch and turkey. In silico analysis confirmed the absence of SP-D-like genes in all 47 published avian genomes. Zebra finch and turkey SP-A1 and SP-A2 sequences, confirmed by PCR of lung homogenates, were compared with sequenced and in silico predicted vertebrate homologs to construct a phylogenetic tree. The collagen domain of avian SP-A1, especially that of zebra finch, was dramatically shorter than that of mammalian SP-A. Amphibian and reptilian genomes also contain avian-like SP-A2 protein sequences with a collagen domain. NCBI Gnomon-predicted avian and alligator SP-A2 proteins all lacked the collagen domain completely. Both avian SP-A1 and SP-A2 sequences form separate clades, which are most closely related to their closest relatives, the alligators. The C-terminal carbohydrate recognition domain (CRD) of zebra finch SP-A1 was structurally almost identical to that of rat SP-A. In fact, the CRD of SP-A is highly conserved among all the vertebrates. Birds retained a truncated version of mammalian type SP-A1 as well as a non-collagenous C-type lectin, designated SP-A2, while losing the large collagenous SP-D lectin, reflecting their evolutionary trajectory towards a unidirectional respiratory system. In the context of zoonotic infections, how these evolutionary changes affect avian pulmonary surface protection is not clear.

Lung infection of avian pathogenic Escherichia coli co-upregulates the expression of cSP-A and cLL in chickens

The host innate defense-pathogen interaction in the lung has always been a topic of concern. The respiratory tract is a common entry route for Avian pathogenic Escherichia coli (APEC). Chicken surfactant protein A (cSP-A) and chicken lung lectin (cLL) can bind to the carbohydrate moieties of various microorganisms. Despite their detection in chickens, their role in the innate immune response is largely unknown. This study aimed to examine whether the expression levels of cSP-A and cLL in the chicken respiratory system were affected by APEC infection. A lung colonization model was established in vivo using 5-day-old specific-pathogen-free chickens infected intratracheally with APEC. The chickens were euthanized 12 h post-infection (hpi) and 1-3 days post-infection (dpi) to detect various indicators. The results of quantitative reverse transcription-polymerase chain reaction and fluorescence multiplex immunohistochemical staining showed that the mRNA and protein expression levels of cSP-A and cLL in the lung and trachea were significantly co-upregulated at 2dpi.Transcriptome RNA-sequencing analysis indicated that the inoculation with APEC AE17 at 2 dpi resulted in differential gene expression of approximately 810 genes compared with control birds, but only a few genes were expressed with astatistically significant ≧2-fold difference. cLL and cSP-A were among the significantly upregulated genes involved in innate immunity. These findings indicated that cSP-A and cLL might play an important role in lung innate host defense against APEC infection at the early stage.

Co-expression of surfactant protein A and chicken lung lectin in chicken respiratory system

Chicken surfactant protein A (cSP-A) and chicken lung lectin (cLL) are C-type lectins that play important roles in pulmonary host defense responses. Herein, we explored the localization of cSP-A and cLL in the chicken respiratory system. Six tissues from 30-days-old SPF chickens were used to quantify the expression of cSP-A and cLL using the quantitative real-time reverse transcriptional polymerase chain reaction (qRT-PCR) and fluorescence multiplex immunohistochemistry staining (fluorescence mIHC staining). Results showed that cSP-A and cLL mRNA were highly expressed in lungs compared to other tissues. cSP-A mRNA expression levels in all tissues were higher compared with cLL expression levels as analyzed using qRT-PCR. Fluorescence mIHC co-expression of cSP-A and cLL were mainly detected in lung parabronchial epithelia, and mucosal epithelia of larynx, trachea, syrinx, bronchus and air sac, with cSP-A showing a stronger positive staining compared with cLL. cLL is expressed on both mucosal surfaces, some individual lung epithelial cells and cartilage cells, while cSP-A is mainly restricted to mucosal surfaces of the respiratory tract. These histological findings may be useful for understanding the biological significance of this pulmonary lectins in future studies.

Co-infection of H9N2 subtype avian influenza virus and infectious bronchitis virus decreases SP-A expression level in chickens

Chicken surfactant protein A (cSP-A) is a collectin believed to play an important role in antiviral immunity. However, cSP-A expression in the respiratory tract of chickens after viral co-infection remains unclear. The aim of this study was the detection and characterization of cSP-A in co-infected chickens. For this purpose, four-week-old specific pathogen-free (SPF) chickens were divided into five groups and inoculated intranasally with H9N2 subtype avian influenza virus (AIV), infectious bronchitis virus (IBV), or Newcastle disease virus (NDV). Chickens were sacrificed at three days post inoculation, and the lung, trachea, and air sac samples were taken to determine histological changes and expression levels of cSP-A mRNA and cSP-A protein. The cSP-A mRNA and its protein were detected separately using real-time quantitative reverse transcriptional polymerase chain reaction (qRT-PCR), a sandwich enzyme-linked immunosorbent assay (S-ELISA), and an immunohistochemistry assay (IHC). In comparison, for the PBS group as the negative group and the NDV-infected group as the positive group, the histological changes showed that the lesions of the AIV+ IBV co-infected group were more serious compared to the AIV-infected group and the IBV-infected group. Consequently, the expression level of cSP-A in the AIV+IBV co-infected group significantly decreased when compared to the AIV-infected group and the IBV-infected group by qRT-PCR, ELISA, and IHC analysis. The mechanism of the downregulation of SP-A expression level will be addressed in future.

Developmental regulation of chicken surfactant protein A and its localization in lung

Surfactant Protein A (SP-A) is a collagenous C-type lectin (collectin) that plays an important role in the early stage of the host immune response. In chicken, SP-A (cSP-A) is expressed as a 26 kDa glycosylated protein in the lung. Using immunohistochemistry, cSP-A protein was detected mainly in the lung lining fluid covering the parabronchial epithelia. Specific cSP-A producing epithelial cells, resembling mammalian type II cells, were identified in the parabronchi. Gene expression of cSP-A markedly increased from embryonic day 14 onwards until the time of hatch, comparable to the SP-A homologue chicken lung lectin, while mannan binding lectin and collectins CL-L1 and CL-K1 only showed slightly changed expression during development. cSP-A protein could be detected as early as ED 18 in lung tissue using Western blotting, and expression increased steadily until day 28 post-hatch. Our observations are a first step towards understanding the role of this protein in vivo.