Human Surfactant Protein D as an Innate Immune Surveillance Molecule

Abstract

Surfactant protein D (SP-D) belongs to the family called Collectins (collagen-containing lectins). It has a primary structure characterised by an N-terminal cysteine-rich region, triple-helical collagen region composed of Gly-X-Y repeats (where X and Y can be any amino acid), an a-helical coiled-coil neck region, and a C-terminal carbohydrate recognition domain (CRD). The primary subunit oligomerizes to form a trimeric structure that can further acquire a cruciform organization and multimers (dodecamers). SP-D is a potent innate immune molecule whose presence in the lungs as well as at a range mucosal surfaces and extrapulmonary tissues allows immune surveillance against pathogens, apoptotic/necrotic cells, allergens, and cancer cells. A recombinant fragment of human SP-D (rfhSP-D) composed of 8 Gly-X-Y repeats, neck and CRD region, expressed in E. coli, is well known to act against a range of pathogen and allergen challenge in vitro, in vivo and ex vivo. In this thesis, we have examined the interaction between rfhSP-D and two subtypes (pH1N1 and H3N2) of Influenza A Virus (IAV) (Chapter 3). rfhSP-D interacted with haemagglutinin, neuraminidase and matrix protein 1; inhibited their infectivity against lung epithelia cell lines; and suppressed the cytokine storm induced by the viral challenge. The protective role of rfhSP-D against IAV as an entry inhibitor was further validated by the use of pseudotyped lentiviruses containing haemagglutinin and neuraminidase of the two IAV subtypes. Continuing with the theme of innate immune surveillance by SP-D against lung, pancreatic, ovarian, and prostate cancers, we extended on recent studies to breast cancer using HER2 over-expressing (SKBR3), triple-positive (BT474) and triple-negative (BT20) breast cancer cell lines (Chapter 4). rfhSP-D induced apoptosis at 24 h in SKBR3 and BT747 cells (but not in BT20) via intrinsic apoptosis pathway. However, this protective effect of rfhSP-D was fully negated by the presence of hyaluronic acid, a major extracellular matrix component within the tumour microenvironment (Chapter 5). This thesis highlights the therapeutic potential of rfhSP-D in influenza A Virus infection as well as breast cancer and merits pre-clinical trials in murine models.