The involvement of TNF-α and TNF-β as proinflammatory cytokines in lymphocyte-mediated adaptive immunity of Nile tilapia by initiating apoptosis

Highlights

• Nile tilapia TNF-α and -β take part in primary response of adaptive immunity.

• Expression of both TNF is induced upon lymphocyte activation.

• TNF-α and -β trigger the apoptosis of head-kidney leukocytes in Nile tilapia.

• TNF-β promoted the apoptosis by activating caspase-8.

Abstract

Tumor necrosis factors (TNFs) are pleiotropic cytokines with important functions in homeostasis and disease pathogenesis. Recent advances have shown that TNFs are also involved in the regulation of adaptive immune responses. However, the knowledge about how TNF participates in and regulates adaptive immune response in early vertebrates is still limited. In present study, we identified two isoforms of TNF, TNF-α and TNF-β, from Nile tilapia Oreochromis niloticus (On-TNF-α and β). After analyzing the sequence characteristics, we investigated their regulatory roles in adaptive immune response of this fish species. On-TNF-α and β are evolutionarily conserved compare with their homologs from other vertebrates. Both TNFs were distributed in a wide range of tissues in O. niloticus, and with relative higher expression level in gill. After the animals were infected by Streptococcus agalactiae, mRNA levels of On-TNF-α and TNF-β in spleen lymphocytes were significantly upregulated during the primary response stage of adaptive immunity. Meanwhile, both TNF proteins in spleen lymphocytes were also dramatically elevated during the adaptive immune stage after bacterial infection. These results indicate the potential participation of On-TNF-α and TNF-β in adaptive immune response of Nile tilapia. Furthermore, On-TNF-α and β transcripts were obviously augmented, once spleen lymphocytes were activated by T cell-specific mitogen PHA. More importantly, both recombinant On-TNF-α and β could induce the apoptosis of head-kidney leukocytes of Nile tilapia. And On-TNF-β but not On-TNF-α promoted the apoptosis by activating caspase-8 in the target cells. Altogether, our study revealed that TNF-α and TNF-β participated in the lymphocyte-mediated adaptive immune response of Nile tilapia by initiating the apoptosis, and thus shed novel perspective for the regulatory mechanism of adaptive immunity in teleost.

Introduction

Tumor necrosis factor (TNF), a family of cellular signaling protein that involved in systemic inflammation, is one of the most critical pro-inflammatory cytokines, which is necessary for efficient innate and adaptive immune responses. Activity of the TNF was firstly described in 1975 (Carswell et al., 1975), and the molecule was then cloned, purified, and characterized in the mid-1980s (Aggarwal et al., 1985). To date, 19 TNF ligands and 30 TNF receptors (TNFRs) have been identified, respectively (Aggarwal, 2003). TNF can be produced by a variety of cell types, including macrophages, T cells, mast cells, granulocytes, natural killer (NK) cells and dendritic cells (Aggarwal et al., 2012; Herbein, 2000). Nowadays, it has been acknowledged that most TNFs could activate NF-κB signaling, and thus play essential roles in regulating inflammation and the immune response against infection (Gaur and Aggarwal, 2003). However, increasing studies in the past two decades also revealed that TNF members usually served as a “double-edged sword”, because their great contributions to the autoimmunity and inflammatory disorders (Aggarwal et al., 2012; Gaur and Aggarwal, 2003).

In the TNF superfamily, TNF-α and TNF-β are two representative members. Although initially discovered as an anti-tumor molecule, TNF-α has now been linked to an array of diseases regarding neurologic, cardiovascular, pulmonary, autoimmune and metabolic (Aggarwal et al., 2012; Idriss and Naismith, 2000; Mahto et al., 2019). TNF-α is synthesized as a type II transmembrane protein and forms a soluble mature protein following cleavage by TNF-α converting enzyme (TACE) (Aggarwal et al., 2012). Functionally, TNF-α triggers cellular responses via interacting with its transmembrane receptors TNFR1 (Cook et al., 2018). Upon activating by TNF-α, the Death domain of TNFR1 recruits downstream signal molecules and then initiates NF-κB and JNK/MAPK pathway to promote cell proliferation (Tesz et al., 2007; Zhang et al., 2007). On the other hand, TNFR1 can also activate the death induction signal mediated by TNF receptor-associated death domain (TRADD) and Fas-associated death domain (FADD), to activate caspase 3, 6 and 8, and eventually cause cell apoptosis (Sheikh and Huang, 2003; Takada et al., 2007; Wertz, 2014). While TNF-β, also called lymphotoxin-α (LT-α), is mainly produced by activated T and B lymphocytes. TNF-β was originally identified as a lymphocyte product that can exert cytotoxic effects (Naoum et al., 2006). In human, TNF-β shares 56% nucleotide identity and 36% amino acid identity with TNF-α; and these two TNFs, especially their functional domains exhibit similar tertiary and quaternary structures (Ware, 2005). In addition, TNF-β seems to employ the same membrane receptors as TNF-α (Naoum et al., 2006). Despite these similarities with TNF-α, TNF-β possesses its unique characteristics. For example, TNF-β is highly efficient in converting to a soluble form, and is never expressed at the cell surface (Calmon-Hamaty et al., 2011). Meanwhile, unlike the ubiquitous expression of TNF-α, expression of TNF-β is limited to immune cells, such as lymphocytes and macrophages.

In the past decade, increasing attentions have been attracted by the essential roles of TNF superfamily in the immune system. As pro-inflammatory cytokines, TNF-α and TNF-β can activate many kinds of immune cells, and prompt secretion of other cytokines for infection clearance. Simultaneously, TNF members have been proven to be essential for lymphocyte activation and function, and thus play pivotal roles in lymphocyte-mediated adaptive immunity (Pamir et al., 2012; Wang et al., 2020). The absence of TNF causes pathological changes in the development and maintenance of lymphoid organs, which further leads to immune disorders and impaired immune responses against various infections (Koroleva et al., 2016). Additionally, both TNF-α and TNF-β are indispensable for B cell development and antibodies production (Sedger et al., 2002a).

Recent advances have suggested TNF as an evolutionarily ubiquitous cytokine throughout the animal, for example mollusk, fish and mammal (Yang et al., 2011). These years, increasing TNF-α genes have been cloned or identified in a variety of fish species, including bluefin tuna, grass carp, channel catfish and tilapia (Kadowaki et al., 2009; Praveen et al., 2006; Zhang et al., 2012; Zou et al., 2003). Unlike those in mammals, multiple TNF-α isoforms have been identified in teleost fish, for example carp and rainbow trout, due to duplications of the whole genome (Hong et al., 2013b; Milne et al., 2017). And fish TNF-α was further classified into type I and type II TNF-α according to phylogenetic tree (Milne et al., 2017). Moreover, obvious differences regarding the structure and distribution among these isomers, indicates the potential complicate functions of fish TNFs (Hong et al., 2013a; Milne et al., 2017). Fish TNF-α could respond to acute stimulation of LPS, bacteria or viruses, while recombinant TNF-α could activate macrophages, suggesting its participation in the innate immune response (Grayfer et al., 2007; Zhang et al., 2012) (Grayfer et al., 2008; Zhang et al., 2012). Although these advances regarding fish TNFs in regulating innate immune response, whether and how fish TNFs participate in adaptive immunity is still unclear. In present study, we identified two kinds of TNF, TNF-α and TNF-β, from the teleost fish Nile tilapia. After analyzing the sequence, structure, phylogenetic and expression characteristics, their potential involvement and regulatory roles to the adaptive immune response of Nile tilapia were investigated. We hope to provide some helpful knowledges for the roles of TNF in adaptive immune responses of fish, and thus to shed a novel perspective for the evolution of adaptive immune system.