**5. Conclusions and Future Perspectives**

In summary, this review has highlighted the strong connection between the liver and gut in the context of liver disease. Indeed, CLD does not occur in isolation, but is accompanied by disturbances of the complex balance of gastrointestinal microbiota, architecture, and immunity. Similarly, gut microbiota can play a significant role in liver health by altering host metabolism and immunity. While such associations are strong, it is much less certain whether the distinct disease states in CLD and fibrosis stimulate changes in microbiota, or if gut microbiota exacerbate inflammatory and fibrotic progression.

Host, microbiomic, and lifestyle factors that influence intestinal permeability are beginning to be understood, but there is still much to be learned. Specifically, there is poor understanding of the exact mechanisms by which intestinal permeability is altered and the physiological reasons for it. TJ expression and localization are often examined in isolation, while overlooking the numerous additional barriers that prevent microbial translocation in the gut. Antimicrobial peptides, IgA, and mucus abundance and localization are rarely examined in this context, and a better understanding of their regulation in health and disease is warranted.

Lastly, along with public health initiatives aimed at reducing the causative lifestyle factors of fibrosis, i.e., alcohol and obesity, we must focus research on the development of novel PRR-antagonizing therapies. Murine studies have highlighted the contribution of individual PRRs to the development and progression of liver inflammation and fibrosis (as summarized in Figure 2); however, it remains unclear how multiple sensors collectively drive disease and may potentiate each signal. Human trials are pending to examine microbial sensors such as TLR9 and STING as well as inflammasome components to determine their contribution, whether alone or in concert, to fibrosis progression.

**Figure 2.** Gastrointestinal microbes and their contribution to liver inflammation and fibrosis. (**A**) In chronic liver disease, gut microbes and PAMPs can cross the intestinal barrier due to an increase in

intestinal permeability, resulting in their transport into the liver through the hepatic portal vein. (**B1**) Gut-derived antigens are recognized by, and activate HSCs, KCs, and hepatocytes, resulting in the secretion of pro-inflammatory cytokines and chemokines. (**B2**) Pro-inflammatory cytokines produced from KCs and hepatocytes further activate HSCs to further exacerbate fibrogenesis. (**B3**) In addition, activation of STING– and NLRP3–inflammasome pathways in hepatocytes can trigger apoptosis and release of DAMPs to further activate HSCs. (**B4**) Chemokines produced by activated KCs, HSCs, and hepatocytes recruit immune cells such as neutrophils, NK cells, and monocytes to further exacerbate liver inflammation and injury. PAMP: pathogen associated molecular pattern; KC: Kupffer cell; HSC: hepatic stellate cell; NK cell: natural killer cell; PRR: pattern recognition receptor; TLR: Toll-like receptor; NOD: nucleotide-binding oligomerization domain-containing protein 1; NLRP3: nucleotide-binding oligomerization domain, leucine-rich repeat- and pyrin-domain-containing 3; STING: stimulator of interferon genes; DAMP: damage associate molecular pattern.

**Author Contributions:** Writing—original draft preparation (S.A.R., T.H.L., D.C.); writing—review and editing (S.A.R., H.S., G.A.); supervision (S.A.R., G.A.).

**Funding:** This research received no external funding.

**Acknowledgments:** Figures were created with Biorender.com (©BioRender-biorender.com).

**Conflicts of Interest:** The authors declare no conflict of interest.
