Till, J. M. | Search bioRxiv

bioRxiv2024

Strain-level antigen variation facilitates immune evasion in Bacteroides thetaiotaomicron

The T cell receptor (TCR) repertoire of intestinal CD4+ T cells is enriched for specificity towards microbiome-encoded epitopes shared among many microbiome members, providing broad microbial reactivity from a limited pool of cells. These cells actively coordinate mutualistic host-microbiome interactions, yet many epitopes are shared between gut symbionts and closely related pathobionts and pathogens. Given the disparate impacts of these agents on host health, intestinal CD4+ T cells must maintain strain-level discriminatory power to ensure protective immunity while preventing inappropriate responses against symbionts. However, to date, the mechanisms by which this occurs have remained enigmatic. To interrogate this, we leveraged B{theta}OM mice that express a transgenic TCR specific for a BT4295-encoded epitope in B. thetaiotaomicron. While many B. thetaiotaomicron strains potently activated B{theta}OM CD4+ T cells in vitro, strain dnLKV9 escaped recognition. Bioinformatic analyses uncovered two BT4295 homologs in B. thetaiotaomicron-dnLKV9, with each homolog harboring sequence modifications relative to strain VPI-5482, specifically a premature stop codon and a T548S substitution within the epitope. Reconstruction of these variants in B. thetaiotaomicron-VPI-5482{Delta}BT4295 conferred evasion from B{theta}OM CD4+ T cells in vitro to this otherwise permissive strain. Adoptive transfer of B{theta}OM CD4+ T cells to gnotobiotic RAG1-/- colonized with B. thetaiotaomicron harboring these variant BT4295 forms verified the sufficiency of these antigen modifications for evasion of B{theta}OM CD4+ T cells. Collectively, these data uncover the existence of strain-level immune evasion in B. thetaiotaomicron and reveal a mechanism whereby strains evade recognition by CD4+ T cells, facilitating strain-level discrimination in responsiveness to the microbiome.

immunology↗

bioRxiv2024

Identification of strain-specific cues that regulate biofilm formation in Bacteroides thetaiotaomicron

Members of the gut microbiome encounter a barrage of host- and microbe-derived microbiocidal factors that must be overcome to maintain fitness in the intestine. The long-term stability of many gut microbiome strains within the microbiome suggests the existence of strain-specific strategies that have evolved to foster resilience to such insults. Despite this, little is known about the mechanisms that mediate this resistance. Biofilm formation represents one commonly employed defense strategy against stressors like those found in the intestine. Here, we demonstrate strain-level variation in the capacity of the gut symbiont Bacteroides thetaiotaomicron to form biofilms. Despite the potent induction of biofilm formation by purified bile in most strains, we show that the specific bile acid species driving biofilm formation differ among strains, and uncover that a secondary bile-acid, lithocholic acid, and its conjugated forms, potently induce biofilm formation in a strain-specific manner. Additionally, we found that the short-chain fatty acid, acetic acid, could suppress biofilm formation. Thus, our data defines the molecular components of bile that promote biofilm formation in B. thetaiotaomicron and reveals that distinct molecular cues trigger the induction or inhibition of this process. Moreover, we uncover strain-level variation in these responses, thus identifying that both shared and strain-specific determinants govern biofilm formation in this species. ImportanceIn order to thrive within the intestine, it is imperative that gut microbes resist the multitude of insults derived from the host immune system and other microbiome members. As such, they have evolved strategies that ensure their survival within the intestine. We investigated one such strategy, biofilm formation, in Bacteroides thetaiotaomicron, a common member of the human microbiome. We uncovered significant variation in natural biofilm formation in the absence of an overt stimulus among different Bacteroides thetaiotaomicron strains, and revealed that different strains adopted a biofilm lifestyle in response to distinct molecular stimuli. Thus our studies provide novel insights into factors mediating gut symbiont resiliency, revealing strain-specific and shared strategies in these responses. Collectively, our findings underscore the prevalence of strain-level differences that should be factored into our understanding of gut microbiome functions.

microbiology↗