The innate immune system serves as the host's first line of defense against pathogenic invasions. In teleost fish, the complement system, also known as the complement cascade, plays a crucial role in combating infections and clearance of apoptotic cells [1]. It can activate the Membrane Attack Complex (MAC) to eliminate pathogens via the alternative pathway (AP), the classical pathway (CP), or the lectin pathway (LP). Leading to various interconnected biological processes, including inflammation, phagocytosis, lysis, and the adaptive immune response [2]. The complement component 1 (C1) complex in the classical complement pathway is essential for recognizing and binding with antibody–antigen complexes, thereby activating the classical pathway. This complex consists of c1q and four serine protease proenzymes (two units of c1r and two units of c1s). In mammals, C1q constitutes a key subunit of the C1 complex in the classical complement pathway, featuring a sophisticated molecular structure composed of 18 polypeptide chains, which include six each of C1qa, C1qb, and C1qc polypeptide chains. The N-terminus of each polypeptide chain contains a globular structural domain (C1qgD) [3,4]. Zebrafish express C1qa, C1qb, and C1qc proteins, with phylogenetic analyses suggesting that these proteins may have originated from the duplication of a single copy of C1qb gene, emphasizing the pivotal role of c1qb in evolution of the complement system [5]. As a crucial link between innate and acquired immunity, c1q's activity is vital for immune processes like phagocytosis, clearance of apoptotic cells, and immune cell adhesion. It's unique protein structure enables c1q to recognize and bind various ligands such as apoptotic cells, variable phospholipids, and β-amyloid fibrils [6,7]. Teleost's c1q exhibits similar immune functions and genome arrangement to mammals [8].

The silver pomfret (Pampus argenteus) is recognized as an economically significant marine species in several Asian nations, experiencing an upward trend in its industrialization. Our research group has successfully propagated approximately one million juvenile silver pomfret specimens [9]. However, the cultivation and breeding of this species encounter numerous challenges, primarily due to a diverse range of diseases that hinder the production of adult fish [10,11]. This includes significant issues with bacterial infections [12,13]. In 2017, we experienced significant losses when our entire stock of cultured silver pomfret perished between July and August. The infected fish displayed numerous white nodules in their kidneys and livers, with Nocardia seriolae (N. seriolae, NS) identified as the causative agent. Subsequent outbreaks in 2021 and 2023 led to widespread mortality among the silver pomfret populations. N. seriolae, commonly encountered in aquaculture, manifests as a systemic, chronic, and granulomatous disease [14]. The infection primarily spreads through the gills, anus, lateral line, or open wounds, presenting as multiple white nodules on internal organs and skin ulcers [15,16]. Macroscopic granulomatous nodular lesions develop on the parenchyma and organs of diseased fish, severely compromising the immune system, diminishing immune function, and threatening survival [17]. N.seriolae, a facultative intracellular Gram-positive bacterium of the Actinomycetales order, Nocardiaceae family, and Nocardia genus [18], exhibits a unique immune evasion strategy. Its robust and complex cell wall not only facilitates evasion from macrophage phagocytosis but also enables migration from organ to organ through macrophage-phagocytosis transport [19]. As a result, this mechanism leads to the formation of granulomas in fish, encapsulating both N. seriolae and affected tissues. While the precise mechanisms of the interactions between N. seriolae and its host and the pathogen's virulence factors are not fully understood, evidence suggests that N. seriolae may induce apoptosis in host cells [[20], [21], [22]]. Additionally, the cell wall of N. seriolae, containing Lipoteichoic acid (LTA), has immunostimulatory properties and can induce both apoptosis and inflammatory responses [23].

This analysis was based on the transcriptome of N. seriolae-infected samples, focusing specifically complement c1qb. Subsequently, we aimed to elucidate whether complement c1qb could inhibit the apoptosis and inflammatory responses induced by N. seriolae infect in vitro. Our study employed an integrated methodology, incorporating overexpression, RNA interference (RNAi), immunohistochemistry (IHC), and Terminal Deoxynucleotidyl Transferase dUTP Nick End Labeling (TUNEL) assays. In summary, the findings of this study offer valuable insights into the basic research on the response of silver pomfret to N. seriolae infections and provide a preliminary reference for the role of the complement C1 family in the innate immune responses of silver pomfret. Furthermore, these results provide theoretical foundations to address bacterial-related challenges in large-scale aquaculture.