Abstract
Background. Inclusion body myositis (IBM) is an autoimmune disease characterised by intense muscle infiltration by CD8+ T lymphocytes with a predominant terminally differentiated effector (TEMRA) phenotype. Recently, IBM was found to be often associated with a lymphoproliferative disorder of CD8+ T cells known as T cell Large Granular Lymphocytes (T-LGL). T-LGLs represent a spectrum of conditions that range from a lymphoproliferation in response to antigenic stimulation “reactive” to an aggressive leukemia. The incidence of expanded T-LGLs among IBM patients (between 40-60%) suggests that these cells play a distinct, yet unknown role in the pathology of this disease. This study aims to determine the T-LGL phenotypical characteristics and TCR clonal diversity and investigates the clinical implications of T-LGLs in IBM.
Methods. Blood samples collected from 85 IBM patients and an aged-matched group of 56 healthy donors were analysed using flow cytometry for phenotype characterisation. CD8+ T cells isolated from PBMCs collected in each donor group were separated into CD57+ (T-LGL) and CD57- (internal control) and sequenced to determine TCR repertoire diversity in both cell type. Multi-variate analysis of biological and clinical data at the time of sample collection, such as aid-assisted mobility, was used to investigate the impact of T-LGL in disease severity.
Results. 33/85 (39%) of IBM patients and 6/56 (10.7%) of healthy controls (HC) exhibited an elevated number of T-LGLs in blood. Immunophenotyping of the TCRαβCD8+ T-LGL population revealed aberrant expression of surface molecules such as decreased CD5 and increased CD57+, CD56+, and KLRG1+ that are commonly displayed by the natural killer cell lineage. Longitudinal analysis of thirteen T-LGL+ patients showed that this expanded cell population persists over time (average time elapsed between samples=1.5 years). Comparison of the proportion of T-LGL within the CD8 T cell population between muscle and blood samples showed that they preferentially accumulate within the muscle (blood: mean= 41.1%±11.7% vs Muscle mean= 70.1%±12.8%). Blood T-LGLs isolated from 6 IBM and 2 HC donors and submitted to bulk TCR-analysis consistently exhibited polyclonal characteristics, with diverse TCRβ VDJ segment-usage and CDR3 repertoire. Clinically, an increased proportion of IBM T-LGL positive patients required more-assisted mobility aids compared to T-LGL negative patients, even though both groups had experienced IBM symptoms for similar durations (T-LGL+ median=12 years vs T-LGL- median=11 years, p-value=0.99)
Conclusions. Our results demonstrate that a CD8+ T-LGLs lymphoproliferative disorder is more prevalent in IBM patients than in healthy individuals. These cells display a late-differentiated phenotype, characterised by reduced TCR-associated co-receptors and high expression of innate lymphocyte-related surface co-receptors and adhesion molecules, suggesting that these cells may be submitted to TCR-independent regulation. In contrast with previous findings, we found that the peripheral CD8+ T-LGL expansion is polyclonal suggesting their expansion is reactive in nature, yet capable of persisting for years. Nonetheless, the presence of expanded CD8+ T-LGL is associated with increased disease severity in IBM. Further investigations into the clonal diversity of muscle-infiltrating T-LGLs should be evaluated to determine if their expansion is antigen-driven or due to bystander activation.