Introduction Clonally expanded CD8 +lymphocytes harbour somatic mutations in RA patients.¹ Seronegative RA, that often displays milder symptoms and slower disease progression, does not share the genetic linkage to the MHC II locus. Thus, we hypothesise that somatic mutations in the clonal CD8 +lymphocytes could modulate inflammation in these patients.

Objectives Our aim was to validate the concept of somatic mutations as regulators of chronic inflammation.

Methods We collected blood samples from seronegative RA patients (n=8) who displayed unusually aggressive, progressive disease that was refractory to treatment and had led to severe joint erosions.² Flow cytometry was used to screen lymphocyte clonality in both CD4 and CD8 lymphocytes and to enrich the mutated clone. TCR-repertoire analysis was performed by TCRB-deep sequencing. Somatic variants were called from deep sequencing data (panel of 1000 immunological genes and exome sequencing) and the data was confirmed by capillary sequencing.

Results Similarly as in our earlier work, there were more suggestive clonal expansions in CD8 +lymphocytes than in CD4 +lymphocytes. The overall clonality of CD8 +cells did not statistically differ between our seronegative destructive RA cohort and age-matched healthy controls (n=19), seropositive (n=53) or seronegative diagnostic phase patients (n=12).

Sequencing of one of the seronegative RA patients‘ CD4 and CD8 T cells with the targeted gene panel, revealed novel somatic mutations in the CD8 +cell pool. The expanded T cell clone was enriched using flow cytometry and from this clone, 22 mutations were identified in genes that are expressed in healthy CD8 +cells. Eight of these mutations were selected and confirmed using capillary sequencing. Based on computational prediction algorithms, we hypothesise that the identified mutations (in PIK3CG, KPNA1, PLCG2, ITGAE, NKAP genes) may regulate the clonal properties, immunological phenotype and even contribute to the regulation of chronic inflammation.

Conclusions We identified somatic mutations in the clonally expanded CD8 lymphocytes from a patient with unusual, aggressive seronegative RA. More work is warranted to elucidate the biological and clinical consequences of the identified mutations.

References

1. . Savola P, et al. Somatic mutations in clonally expanded cytotoxic T lymphocytes in patients with newly diagnosed rheumatoid arthritis. Nature Communications2017;8:15869.

2. . Nikiphorou E, et al. Long-term outcomes of destructive seronegative (rheumatoid) arthritis: Description of four clinical cases. BMC Musculoskelet Disord2016;17:246.

Disclosure of interest None declared