A subunit vaccine based on Brucella rBP26 induces Th1 immune responses and M1 macrophage activation

Abstract

Brucellosis is a global zoonotic infection caused by Brucella bacteria, which poses a significant burden on society. While transmission prevention is currently the most effective method, the absence of a licenced vaccine for humans necessitates the urgent development of a safe and effective vaccine. Recombinant protein-based subunit vaccines are considered promising options, and in this study, the Brucella BP26 protein is expressed using prokaryotic expression systems. The immune responses are evaluated using the well-established adjuvant CpG-ODN. The results demonstrate that rBP26 supplemented with a CpG adjuvant induces M1 macrophage polarization and stimulates cellular immune responses mediated by Th1 cells and CD8+ T cells. Additionally, it generates high levels of rBP26-specific antibodies in immunized mice. Furthermore, rBP26 immunization activates, proliferates, and produces cytokines in T lymphocytes while also maintaining immune memory for an extended period of time. These findings shed light on the potential biological function of rBP26, which is crucial for understanding brucellosis pathogenesis. Moreover, rBP26 holds promise as an effective subunit vaccine candidate for use in endemic areas.

1. Introduction

Brucellosis, a zoonotic infection caused by Brucella bacteria, poses a significant public health concern, affecting an estimated 3.5 billion people worldwide [1]. Brucella are transmitted primarily through close contact with the blood, feces, urine, and placenta of infected host animals. Infections can result in miscarriages in female animals and orchitis in males, leading to a decrease in reproductive capacity and compromising the quality and safety of animal products [2]. In humans, brucellosis manifests as symptoms such as fever, arthritis, orchitis, hepatitis, encephalomyelitis, and endocarditis [3]. Presently, several live attenuated vaccines, including S19, Rev.1, S2, SR82, and RB51, are widely used [4,5]; however, these vaccines have certain limitations. For instance, some infections and complications may occur in humans, including the risk of miscarriage in pregnant cows [6]. Given the absence of a licenced vaccine for human use, there is an urgent need to develop a safe and effective vaccine suitable for high-risk workers and individuals residing in endemic areas.

With advancements in molecular biology technology and a deeper understanding of the pathogenesis of brucellosis, there is a promising opportunity for the development of new transgenic vaccines to replace conventional vaccines for brucellosis control. Moreover, recombinant protein vaccines based on antigen recognition have gained significant attention and have been extensively studied [7,8]. These vaccines, expressed in either eukaryotic or prokaryotic systems, offer the advantage of inducing high levels of antibodies while addressing safety concerns associated with live vaccines [6]. Researchers have cloned various Brucella outer membrane proteins and explored their immunogenicity in an effort to identify components that can effectively protect target animal species. Notable examples include p39 [9], OMP31 [10], Cu-Zn-SOD [11], L7/L12 [12], OMP2b [13], and TF [14]. These studies aimed to identify specific protein components that could be utilized in the development of protective vaccines.

Among the many outer membrane proteins, BP26 is located in the periplasm of Brucella and has been identified as an important diagnostic antigen for brucellosis. It is highly conserved in the Brucella genus and widely used as a specific marker for diagnosing brucellosis [15‒17]. As a potential vaccine candidate, BP26 can stimulate both humoral and cellular immune responses in the host in combination with other agents, providing protection against Brucella infection [18,19]. However, macrophages, which are the first line of defense against Brucella, play a crucial role in linking innate and adaptive immunity. When being infected with Brucella, macrophages recognize, bind, and internalize the pathogen while also secreting soluble antimicrobial agents and innate immune mediators to defend against the infection. Although several Brucella outer membrane proteins have been shown to activate macrophages to secrete pro-inflammatory cytokines, the function of rBP26 in innate and adaptive immunity, as well as its protective role in host defense against brucellosis, remains largely unknown.