Anti-Filamentous HA monoclonal antibody, clone IZC 353-02

Filamentous hemagglutinin (FHA) is one of the key adhesins in Bordetella pertussis, playing a significant role in the bacteria's virulence and pathogenic mechanisms. As a crucial component of the acellular pertussis vaccine, FHA is central to the interaction between the bacterium and host cells. This protein is a typical example secreted via the Two-Partner Secretion (TPS) pathway, which is common in Gram-negative bacteria. Many important virulence factors, especially large adhesins, are secreted through this mechanism. The TPS system consists of two different proteins: a secreted protein (TpsA) and an outer membrane transporter protein (TpsB). FHA represents a prime example of a TpsA protein, synthesized first as a precursor protein known as FhaB, with a molecular weight of around 370 kDa. Once FhaB is translocated across the cytoplasmic membrane, it undergoes a series of intracellular processing steps, ultimately being cleaved into the bioactive "mature" FHA molecule, which has a molecular weight of about 240 kDa. The structure of FHA is intimately connected to its function. It adopts a β-helix core structure, which contributes not only to its stability but also provides an ideal conformation for attachment to host cell surfaces. The TPS domain of FHA is primarily responsible for initiating the secretion process and forming the β-helix structure, while the C-terminal pre-structural domain regulates FHA maturation. Studies have shown that the deletion of FHA's C-terminus significantly impairs its function within host cells, leading to a reduced ability for the bacteria to colonize. Moreover, the mature C-terminal domain (MCD) is considered the main region involved in FHA's interaction with host cells. Proper folding of this domain is essential for FHA's adhesive function, mediating bacterial adhesion by binding to host receptors. These findings reveal the complex interplay between the structural domains of FHA, suggesting that its multifunctionality extends beyond what was initially understood.

Figure 1. FHA biogenesis (Source: Scheller EV, et al., 2015)

The signal peptide at the N-terminus is first removed during FHA secretion, and then the TPS domain interacts with the outer membrane protein FhaC to enable FHA's translocation across the membrane. The protein gradually folds into a β-helix from the N-terminus to the C-terminus after the N-terminal section of FHA first binds to FhaC. A Brownian ratchet mechanism then transports the protein to the bacterial surface. Proper secretion of FHA depends on the integrity of this β-helix shape. It facilitates effective translocation across the membrane while preserving the functional structure of FHA. The breakdown of the C-terminal pre-structural domain is a crucial stage in the maturation of FHA. The final mature version of FHA is produced by cleaving its C-terminus, which is done by the outer membrane protease SphB1. It is yet unknown, nevertheless, if FHA must be fully liberated from the bacterial surface in order to carry out its role during infection. At first, it was thought that FHA mostly helped Bordetella pertussis adheres to host cells, especially respiratory epithelial cells. However, increasing evidence suggests that FHA may have a broader role in the pathogenic process. For example, FHA has been found to modulate the host immune response, with Bordetella pertussis using FHA to suppress host immune cell activity and reduce inflammation, thus promoting bacterial persistence in the host. Experimental studies show that mutant strains of Bordetella pertussis lacking FHA have significantly reduced infection capabilities in animal models, especially in terms of colonization and persistence in the lower respiratory tract. FHA plays a crucial role not only in the initial adhesion process but also in modulating the host immune response, potentially allowing the bacteria to escape immune surveillance. However, the precise functions of FHA in vivo are not yet fully understood. While its release is considered part of its function, studies have shown that even unreleased FHA can still participate in bacterial adhesion and infection, suggesting that FHA's role extends beyond what has been observed in vitro, involving a more complex host-pathogen interaction network.

FHA has been used extensively to prevent pertussis as a major part of the acellular pertussis vaccine. However, declining vaccine-induced immunity and pathogen genetic variation have been blamed for the recent increase in pertussis cases worldwide. Scientists are reevaluating the roles of FHA and other virulence factors in vaccine design as a result of this incident, which has raised concerns about the vaccine's effectiveness. Some studies suggest that genetic diversity in FHA may influence its expression and function in different Bordetella pertussis strains. Recent surveillance data from the United States indicate that some circulating strains have shown deletions in FHA and Prn (another important adhesin), which may be linked to vaccine pressure. This underscores the importance of further research into FHA's role in vaccine-induced immunity. Overall, FHA, as a significant virulence factor, highlights many yet-to-be-fully-understood biological mechanisms in bacterial pathogenesis due to its complex secretion process, multifunctionality, and interactions with the host immune system. Future research should continue to explore the specific functions of FHA during infection, particularly its multi-layered role in bacterial-host interactions. This will not only help uncover the pathogenic mechanisms of pertussis but also provide theoretical foundations for developing more effective vaccines and therapeutic interventions.

Anti-HA antibody
Anti-filamentous hemagglutinin antibody
HA monoclonal antibody