Table 1.

Characteristic features of included studies and the reported on the expression of programmed cell death 1 (PD-1) on T-cells in patients with type 2 diabetes (T2D).

Study	Country	Study size	Male, n (%)	Age, y	T-cell subsets reported	Main findings
Shi et al, 2013[18]	China	90 participants (42 T2D and 48 controls)	Not reported	51.46 ± 10.68	CD4+CD28–	The expression of PD-1 on CD4+CD28- T-cell subset was increased in T2D patients when compared to healthy controls. Moreover, the upregulation of PD-1 on these T-cells was associated with the development of atherosclerotic macrovascular diseases.
Fujisawa et al, 2015[28]	Japan	48 participants (19 T2D and 29 controls)	24 (50)	49 ± 11.82	CD4+	The levels of PD-1 expression on T-helper cells were comparable between individuals with T2D versus healthy controls.
Jia et al, 2016[9]	China	130 participants (80 T2D and 50 controls)	70 (54)	61 ± 4.10	CD4+ and CD8+	The expression of PD-1 on both CD4+ and CD8+ was increased in T2D when compared with healthy controls. In addition, the upregulation of PD-1 on T-cells positively correlated with the levels of C-reactive protein (CRP), an inflammation marker.
Nyambuya et al, 2018[29]	South Africa	69 participants (34 T2D and 35 controls)	10 (14)	54.48 ± 4.45	CD4+	There was no difference in the expression of PD-1 on T-cells in T2D patients and healthy controls. Moreover, there was no correlation between the expression of PD-1 on T-cell and glucose metabolic profile. However, T2D patients had increased levels of inflammation.
Sun et al, 2019[19]	China	43 participants (23 T2D and 20 controls)	20 (47)	57.47 ± 9.03	CD4+ and CD8+	CD4+ and CD8+ T-cells from individuals with T2D expressed lower levels of PD-1 when compared with healthy controls. However, no correlation was found between PD-1 expression and glucose metabolic profiles.