Role of complement proteases in Streptococcus pathogenesis in fish

Abstract

Streptococcal pathogens, particularly Streptococcus iniae, are emerging as a major cause of meningitis and septicaemia in a broad range of farmed and wild fish species, and are occasionally zoonotic. The major virulence factor in fish pathogenic streptococci is a highly variable antigenic polysaccharide capsule, which has resulted in serotype switching and reinfection of previously vaccinated fish. Therefore there is an urgent need to investigate more highly conserved protein-based critical virulence factors as targets for new vaccines.Complement proteases C5a peptidase and C3 protease of S. iniae were investigated in the present study to determine their role in virulence, as they are both secreted or surface associated and may represent potential targets for vaccination. My research shows that the C5a peptidase (scpI) and C3 protease (cppA) are inversely regulated with capsular polysaccharide. This coordinate regulation allows the capsule to be down-regulated to permit attachment to hydrophobic surfaces which may be essential for invasion of the Central Nervous System (CNS) and rapid onset of meningitis, whilst the proteases are upregulated to protect the bacteria from complement-enhanced killing by neutrophils in barramundi whole blood assays. Knockout mutants deficient in each of the genes revealed reduced growth in whole blood, serum and antiserum. Moreover, these mutants revealed a role for ScpI and CppA in the interruption of the classical pathway since they were susceptible to the antiserum, which contained specific antibodies and complement proteins activating the classical pathway. Additional work to determine whether blood and serum resistance can be rescued by complementing the knockout mutants is required. To explore environmental factors in the host that may drive coordinate regulation of scpI and cppA with capsular polysaccharide synthesis protein E (cpsE) iron limitation was investigated.Iron stress appeared to be as one of the triggers for reduction the expression of capsule and coordinate increase in complement proteases expression. Future work should elucidate further environmental cues in the host that regulate expression of these genes, as it is likely to be complex and multifactorial. Together, these data implicate complement proteases in survival and dissemination in the host and therefore warrant further investigation as conserved protein targets for vaccines for farmed fish.

Comparative genomics analyses of fish pathogenic Streptococcus spp. isolated from tilapia and flounder

“…Virulence genes encoded for extracellular enzyme (hylB and eno) and protease synthesis (cppA, htrA/degP) were found in the genome sequences of fish-derived strains along with human pathogens of S. agalactiae (Kayansamruaj et al, 2015). Furthermore, C3 protease (cppA) was found in the pathogenic S. iniae (Nguyen, 2015). Trigger factor (tig/ropA) act as a chaperone and is involved in protein exportation and ropA is an essential factor for the secretion and maturation of the cysteine protease (Lyon & Caparon, 2003).…”