Abstract
The lectin pathway of the complement system is one of the main components of innate immunity, which plays a pivotal role in the defense against infectious microorganisms and maintains immune homeostasis. However, its control mechanisms remain unclear in teleost fish. In this study, we described the identification and functional characterization of a mannose-binding lectin associated protein MAp34 (OnMAp34) from Nile tilapia (Oreochromis niloticus) at molecular, cellular, and protein levels. The open reading frame (ORF) of OnMAp34 is 918 bp of nucleotide sequence encoding a polypeptide of 305 amino acids. The deduced amino acid sequence has three characteristic structures, including two C1r/C1s-Uegf-BMP domains (CUB) and one epidermal growth factor domain (EGF). Expression analysis revealed that the OnMAp34 was highly expressed in the liver and widely existed in other examined tissues. In addition, the mRNA and protein expression levels of OnMAp34 were remarkably altered upon infection with Streptococcus agalactiae and Aeromonas hydrophila in vivo and in vitro. Further, we found that the OnMAp34 could participate in the non-specific cellular immune defense, including the regulation of inflammation, migration, and enhancement of phagocytosis of monocytes/macrophages. Moreover, the OnMAp34 could compete with OnMASPs to combine OnMBL and inhibit the lectin pathway of complement activation. Overall, our results provide new insights into the understanding of MAp34 as a potent regulator in the lectin complement pathway and non-specific cell immunity in an early vertebrate.
Highlights
Complement is the central part of innate immunity that plays a key role in the defense against pathogens and homeostasis [1]
(r)OnMAp34, (r)OnMAp44, (r)OnC1INH, or Trx was added to diluted Nile tilapia serum in an mannose-binding lectin (MBL)-binding buffer (20 mM Tris-HCl, 10 mM CaCl2, 1 M NaCl, 0.05%, Triton X-100, 0.1% BAS, pH 7.4); the mixtures were incubated in mannan-coated plates for 3 h to form lectin–MBL-associated serine proteases (MASPs) complexes
The results indicated that the (r)OnMAp34, (r)OnMAp44, and (r)OnMASP-1 could bind to the OnMBL in a concentration-dependent manner (Figure 6A)
Summary
Complement is the central part of innate immunity that plays a key role in the defense against pathogens and homeostasis [1]. The LP is the key to inducing immune response and plays a vital role in the host resistance to pathogen infection. MASP-1 can play roles in a variety of biological effects, including coagulation, enzymatic hydrolysis of factor XIII and fibrinogen, cell activation, and inflammation regulation [7, 22, 23]. We reported the expression and function of a mannose-binding lectin/ficolin-associated protein from O. niloticus, which was denominated OnMAp34. Our results provide new insights into the understanding of MAp34 as an important regulator in the lectin complement pathway and nonspecific cell immunity in an early vertebrate
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