Exploring Host-Commensal Interactions in the Respiratory Tract

Abstract

Commensal microbes are currently in the limelight in biomedical research because they play an important role in health and disease. Humans harbor an enormous diversity of commensals in various parts of the body, including the gastrointestinal and respiratory tracts. Advancement in metagenomic and other omic approaches, and development of suitable animal models have provided an unprecedented appreciation into the diversity of commensals, and the intricacies of their intimate communication with the host immune system. Most studies have focused on the host-commensal interaction in the gut, while less is known on this relationship in other sites of the body, such as the respiratory tract. In this article, we review emerging data from human and animal studies on the host responses to respiratory commensals, immune cross-reactivity between commensals and pathogens, and use of commensals as a vaccine delivery system. A better understanding of the delicate interplay between commensals and host may aid in efforts to develop effective vaccines and therapeutics.

Keywords: commensal; host; immunity; lungs; vaccine.

Figure 1

Use of commensals as a prophylactic strategy against respiratory pathogens. Commensals showing high homology with pathogenic species have the potential to induce cross-reactive immune responses and to act as a vaccine delivery system. Commensal Streptococcus mitis induces cross-reactive antibody (Ab) and T cell (T) responses against pathogen Streptococcus pneumoniae. Production of bacteriocins (B) and hydrogen peroxide (H₂O₂) by S. mitis elicits direct protection against pathogens like S. pneumoniae. In addition, S. mitis has been used as a potent vector (V) for delivery of heterologous bacterial and viral antigens, which confers robust and antigen-specific immunity against respiratory infections. Scanning electron microscopy images of S. mitis NCTC12261 (left), and S. pneumoniae TIGR4 (right).