Other innate immunity components in fish mucus
There are also a few important components of fish epidermal mucus which play an important role in innate immunity of fishes such as C-reactive proteins (CRPs), transferrin, ALP, complement proteins, etc. CRPs belong to a family of multifunctional proteins (pentraxins) and are capable of binding various ligands in a Ca2+ dependent binding affinity. These are characterized by their cyclic pentameric structure and high sequence similarity (Gewurz et al., 1995). The main ligand for CRP includes the phosphoryl choline moiety of pneumococcal C-polysachcharides (CPS) and phospholipids.
C-reactive protein takes part in innate immune defense through activation of the complement pathways and plays an important role in the recognition and clearance of apoptotic cells (Nauta et al., 2003). It can also act as opsonin and can cause the precipitation and agglutination of macromolecules or microorganisms with surface phophorylcholine.
Also, increased levels of CRP were found in the serum and mucus of Tilapia mossambica after induced physical injury which suggested its role in early detection of diseases (Ramos and Smith, 1978). Pentraxin-like molecules have been isolated from a number of teleost fish species including Atlantic salmon (S. salar), common wolfish (Anarhichas lupus), cod (Gadus morhua), halibut (H. hippoglossus) and Indian carp (Catla catla) (Lund and Olafsen, 2003).
Transferrin, an iron binding glycoprotein plays a significant role in transportation of iron between the sites of absorption, storage and utilization in vertebrates (Putnam, 1975). All organisms including microbes and parasites require iron for growth and it is considered to be an essential element in the establishment of infection (Sussman, 1974). Hence, transferrin plays an important role in innate defense mechanism of fish by binding iron and reducing its availability to the invading pathogens by chelating iron.
It helps in countering the growth of pathogens until immune system can respond. The range of molecular weights of transferrins occurring in fishes is 70-80 kDa (Alexander and Ingram, 1992). Its presence as a non-specific immunity component has been reported in skin mucus of olive flounder (Paralichthys olivaceus) (Palaksha et al., 2008). Increased levels of cleaved transferrin fragments were detected after infection with sea lice (L. salmonis) in channel catfish (Ourth et al., 1991) and Atlantic salmon (Easy and Ross, 2009) as part of the fish’s immune response to sea lice infection.
Further, presence of transferrin was also reported in sea bass and sea bream (Cordero et al., 2015; Sanahuja and Ibarz, 2015). Alkaline phosphatase, a lysosomal enzyme present in epidermal skin mucus of fish has been shown to act as an antibacterial agent because of its hydrolytic activity (Dash et al., 2011; Dash et al., 2014; Guardiola et al., 2014). The level of ALP increased in the fish during skin regeneration, in the initial stages of wound healing, stress and parasitic infection (Rai and Mittal, 1983; Iger and Abraham, 1990, 1997; Ross et al., 2000).
In the case of Rainbow trout, coho salmon and Atlantic salmon ALP was undetected in the skin mucus unless the fish were transferred from freshwater to marine water (Fast et al., 2002).
Studies have demonstrated several stressors such as acidity, thermal elevation, polluted water and distilled water caused increases in the number of alkaline phosphatase-positive Rodlet cells in the skin of Rainbow trout (Iger and Abraham, 1997). Rodlets contain ALP at their periphery and peroxidase activity at their cores which contributes to the non-specific defense mechanisms of skin of fish. Complement system constitutes another important component of innate response in mucus.
It contains a group of protein and non-protein components that play a major role in both innate and adaptive immunity. It contains approximately 35 plasma and membrane-bound proteins that mediate a chain reaction of proteolysis which results in the elimination of invading microorganisms (Boshra et al., 2006). Important complement proteins like C3, C7 and C1q have been reported in skin mucosa of Atlantic halibut (H. hippoglossus), grass carp (Ctenopharyngodon idella) and Siberian sturgeon (Acipenser baerii) respectively (Magnadottir et al., 2005; Shen et al., 2012; Fan et al., 2015).