**6. Conclusions**

Molecular biological approaches have enhanced our understanding of the contribution of Epstein-Barr virus to the pathogenesis of Sjögren's syndrome. The involvement of HTLV-1 has been thought to vary among endemic areas, whereas EBV infection is ubiquitously observed regardless of regional characteristics. In addition, the aging of HTLV-1-infected populations might influence the prevalence of HTLV-1-seropositive SS. To determine the direct contribution of both EBV and HTLV-1 in the pathogenesis of SS, the integration of epidemiological and experimental studies including virological and molecular immunological approaches is strongly desired.

**Author Contributions:** All authors were involved in drafting the article or revising it critically for important intellectual content, and all authors approved the final version for publication. H.N. has full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. Study conception and design: H.N. Interpretation and critical reading of this review: H.N., T.S., A.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** No external funding was involved in this review.

**Acknowledgments:** This review was supported in part by the JSPS KAKENHI (grant no. JP19K08884).

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