Type 1 diabetes is a T cell mediated autoimmune disease, often presented in children. Development of T1D is a chronic progression, wherein memory CD4+ T cells (CD4+ Tm) are believed to play a pivotal role in mediating autoimmune responses. It has been proposed that dysfunction in effector memory cells may contribute to autoimmunity in T1D. Using a unique cohort of age- and gender-matched children with T1D, their non-diabetic siblings and unrelated healthy control children, we undertook a detailed immunological study, including circulating CD4+ Tm cell counts, and activation, proliferation, effector responses and apoptosis in reactivated CD4+ Tm cells during TCR stimulation induced by anti-CD3/anti-CD28 mAb.
We found T1D was associated with a reduced circulating CD4+ Tm cell count, and a similar reduction in circulating CD4+ Tm cells was also evident in the siblings. In both children with T1D and siblings, the reduced circulating CD4+ Tm cell counts were resulted from a decrease in the central memory CD4+ T cells. In reactivated CD4+ Tm cells, no intergroup difference in proliferation was found across the cohorts. Reactivated CD4+ Tm cells from children with T1D and siblings acquired CD25+ phenotype at a higher proportion when compared to controls. In addition, reactivated CD4+ Tm cells from children with T1D and siblings were capable of mounting exaggerated effector responses through effective transitioning from central memory (CD62L+) to effector memory (CD62L-) phenotype. The exaggerated effector responses were characterised by elevated IFN-γ and IL-10 production in T1D patients and elevated IFN-γ production in the siblings. These exaggerated Tem responses were also accompanied by an increase in Fas-independent mitochondria-associated apoptosis, which eliminated approximately 50% of the CD4+ Tem cells in T1D and siblings, but only one third of CD4+ Tem cells in controls. The increased apoptotic response in reactivated CD4+ Tm cells from children with T1D inversely correlates with their reduced blood CD4+ Tm counts, suggesting that the susceptibility to reactivation-induced apoptosis may be associated with an altered in vivo dynamics of blood CD4+ Tm cells in T1D.
In conclusion, this study has revealed a novel hyperactive phenotype of reactivated CD4+ Tm cells in T1D patients and their siblings. In these cells, an exaggerated activation and Tem phenotype were counterbalanced by heightened apoptotic response. Whether the exaggerated activation, Tem phenotype and apoptosis herein induced in reactivated CD4+ Tm cells by polyclonal TCR stimulation also operates across other T cell lineages (e.g. CD8+ T cell lineages) as well as in response to islet-specific antigen challenge is warranted in further studies. Nonetheless, this study provides new insights for further defining mechanisms underlying T1D pathophysiology, and illuminates specific immune cell lineages, biologically relevant surface receptors, cytokines, and biochemical pathways for therapeutic targeting.