The PD-1/PD-L1 axis is essential for maintaining immunological homeostasis in many organs, including pancreatic tissues. In 1.8% of cancer patients treated with anti-PD-1 antibodies, blocking this route resulted in acute or chronic pancreatitis. In addition, evidence revealed that this inhibitory pathway is particularly significant in maintaining immunological tolerance towards insulin-producing pancreatic β cells and, hence, plays a significant role in protection against T1DM [6, 18]. Studies on transgenic mice have highlighted the importance of an intact PD-1/PD-L1 axis in protecting against organ-specific autoimmune diseases. Compared to controls, transgenic mice have revealed a decrease in the severity of insulitis, delay in the onset of the disease, and reduction in the overall diabetes incidence. In addition, the transgenic mice demonstrated a different nature of lymphocytes with lower proliferative potentials than the controls [19].
In the present study, three SNPs within the PD-1/PD-L1pathway were investigated to detect a possible involvement in developing T1DM in pediatric patients. In terms of PD-1 rs34819629 SNP, the current investigation revealed that all patients and healthy control subjects inherited the same genotype (GG genotype). Due to the current results, we could not detect a correlation between PD-1 rs34819629 and the risk of occurrence of T1DM. Our findings are consistent with those of Qian et al. [20], who found no association between T1DM and the PD-1 rs34819629 [20]. In contrast, several other studies have revealed a correlation between the SNP and other autoimmune diseases, including allergic bronchial asthma, systemic lupus erythematosus, and ankylosing spondylitis [21, 22]. The absence of association could be attributed to the differences in the pathogenesis between T1DM and other autoimmune diseases [20].
In terms of PD-L1 rs 4143815 SNP, the investigation revealed that the GG genotype and the major G allele were more frequent within the patients’ group than in the control subjects. However, there was no significant difference between the groups (p value of 0.123 and 0.071, respectively). Similarly, in the stratification of the diabetic group, according to three genetic models to estimate the risk of T1DM in the presence of PD-L1 rs4143815, the results demonstrated that the diabetic group had a greater level of the major G allele in the three genetic models (general, dominant, and recessive) compared to the control group. Nevertheless, the difference did not reach a significance level (p = 0.078 and p = 0.055 for the general genetic model, p = 0.061 for the dominant model, and p = 0.169 for the recessive model, respectively). The findings of this study are consistent with those of Pizzaro et al. [23], who indicated that the significant G allele was more significant in the diabetic group than in the control group, but the difference between the two groups was not statistically significant (p = 0.058) [23]. In contrast, the results of Qian et al. indicated that PD-L1 rs4143815 was significantly associated with T1DM as the inheritance of the G allele was significantly associated with autoantibodies against islets antigens are used for T1DM diagnosis [20].
With respect to the potential association of PD-L1 rs2297137 with T1DM, our findings indicate the presence of a potential association between the SNP and the T1DM as the results revealed that the GG genotype and the major G allele are the most frequently inherited among the diabetic group in comparison to the control group (p = 0.018 and p = 0.005). Moreover, on stratification of the diabetic and control groups according to three genetic models (the general, dominant and recessive genetic models) to estimate the possible risk associated with the presence of rs2297137 and T1DM, the results demonstrated that in the recessive genetic model, the major G allele was found to be strongly related with the risk of T1DM (p = 0.007).
The present study's findings are consistent with those of Pizzaro et al. [23], who reported a significant difference in allelic distribution, with the G allele being the most prevalent among the diabetic group compared to the control group (p = 0.035) [23, 24]. In contrast, Qian et al. found no correlation between PD-L1 rs2297317 and T1DM [20]. HbA1c is a significant indicator of chronic hyperglycemia and correlates with the possibility of long-term problems of diabetes because it is a helpful measure of glycemic control [25]. The current study found that HbA1c levels were significantly related to both recessive genetic models in PD-L1 rs4143815 and rs22937137 (p values < 0.001 and 0.001, respectively). Consequently, this implies that both PD-L1rs4143815 and rs22937137 may be related to the risk of chronic hyperglycemia and long-term diabetes complications.
Type one diabetes can affect anyone at any age, but it is most common in children and young people. Since the incidence of T1DM is substantially lower in young adults than in children, most investigations have focused on children under the age of 15 [26]. Since 1938, diabetes mellitus has increased by around 2%, with no gender differences in children aged 0–14, although males aged 15–39 years have a two-fold higher prevalence than females [27]. This finding is consistent with the current study results, as comparing the relation of rs4143815 and rs2297137 with the age of onset of the disease and gender variation did not reveal a significant difference in the three genetic models. Furthermore, a comparison of the BMI of the diabetic group and rs4143815 and rs2297137 in the three genetic models revealed no significant relation.
The current study results on the association of PD-L1 rs2297317 and T1DM demonstrate the importance of PD-L1 as part of the PD-1/PD-L1 pathway in protection against T1DM. Evidence indicates the importance of an intact PD-1/PD-L1 axis in protecting against autoimmune diseases, especially organ-specific ones. This inhibitory pathway contributes to protecting against T1DM in humans and mice. T1DM develops in nonobese diabetic mice with PD-1 or PD-L1 deficiency [28, 29].
Preclinical studies have shown that in prediabetic nonobese diabetic mice, PD-1 or PD-L1 blockade was responsible for developing T1DM [5]. Other studies have shown that PD-L1 plays a protective role against autoimmune diabetes [30]. It has been hypothesized that PD-L1 expression on parenchymal cells may inhibit autoreactive CD4+T cell tissue destruction and effector cytokine production and hence plays a protective role against autoimmune DM [31, 32]. Moreover, PD-L1 blockade has led to the inhibition of T cell migration, extended T cell and dendritic cell engagement, increased the production of cytokines by T cells, augmented T-cell receptor (TCR) signaling, and abrogated peripheral tolerance [33]. Therefore, these findings may explain the results of the current study's findings and the impact of PD-L1 rs 2297137 on the risk of T1DM.
The discrepancies between our study's findings and other studies that investigated the association between the PD-1/PD-L1 pathway and T1DM could be explained by several factors, including variations in the analytical methods utilized in the research, discrepancies in the ethnic origins of the examined population, and difference in the sample size and characteristics of the studied populations.
Limitations
The present study has some limitations; first, pancreatic antibodies were not measured as anti-GAD65 antibodies, insulin autoantibodies, or islet cell antibodies to determine their correlation with susceptibility to the studied genotypes. Second, the current study could not interpret the correlation between genotype and allele frequency distribution with the prevalence of diabetic ketoacidosis at presentation. In order to determine the effect of the SNPs on gene expression, the expression pattern of the genes should be determined, and detection of the serum level of PD-1 and PDL-1 should be considered to correlate the results. In addition, the sample size was limited. Future studies with a larger number of patients are required to generalize the results.
