INTRODUCTION
Epidemiological data clearly show a global increase in the incidence and prevalence of nontuberculous mycobacterial (NTM) disease (
1–3). In South Korea, several studies have consistently reported a substantially increasing trend of NTM disease in the last 15 years (
1,
4). Several factors, such as technical advances in the isolation and identification of NTM from clinical specimens, wider recognition of NTM as a human pathogen, overall population aging, decreased immune function caused by various medical treatments for cancer or rheumatologic disease, and frequent exposure to environmental mycobacteria, have been implicated as possible causes of the increase in NTM disease in the past decades (
5–8).
Diabetes mellitus (DM) is one of the most common diseases in South Korea, and the average annual prevalence of DM and impaired fasting glucose (IFG) in Korean adults aged ≥20 years is 13.8% and 28%, respectively (
9). Oh et al. reported that most South Korean diabetes patients had type 2 DM (T2DM), which accounts for 88.0% of all cases of diabetes (
10). DM increases the severity, incidence, and complications of infectious diseases (
11), as DM impairs macrophage or T-cell function and cytokine production (
12,
13). Tuberculosis is an important infectious disease that is associated with diabetes (
14). Compared with those without diabetes, patients with diabetes have a severalfold higher risk for tuberculosis (
15,
16). Based on an analysis of health insurance data, Golub et al. reported that South Korean DM patients had a higher risk for tuberculosis than those without DM (
17).
Patients with NTM disease and tuberculosis have several similar clinical characteristics, including clinical symptoms, radiographic findings, and treatment regimens (
18–20); however, studies of the impact of DM on incident NTM disease are limited (
21). Using a nationwide longitudinal cohort in South Korea, we aimed to assess whether T2DM, the major form of DM in Korean adults, is a significant risk factor for NTM disease. Furthermore, we investigated whether the NTM risk differs in the presence of diabetes-related complications.
DISCUSSION
The incidence and prevalence of NTM disease have recently increased worldwide (
1–3), and DM is one of the most common chronic diseases whose prevalence has increased in the last several decades (
22). However, few studies have investigated the relationship between DM and the incidence of NTM disease so far (
21). In the present study, through analysis of NTM-naive matched cohorts from the data of a national population-based cohort which represents 2.2% of the total South Korean population, we assessed whether patients with T2DM are at a higher risk for incident NTM disease. The key findings were that (i) T2DM alone is not a statistically significant risk factor for NTM disease, but (ii) however, T2DM significantly increases the hazard of NTM disease development in the case of ≥2 diabetes-related complications.
In tuberculosis, several studies have elucidated that diabetes and diabetes control status affect several aspects, including clinical manifestation, treatment outcome, and recurrence as well as the development of tuberculosis (
15,
16,
23,
24). In contrast, few studies have investigated the impact of diabetes on the occurrence or outcome of NTM disease except one recent study which showed that patients with DM are at higher risk for NTM-caused disease (
21). In that study, Wang et al. reported that the incidence of NTM disease in patients with type 1 and T2DM was 1.43-fold greater than that in patients without DM after analyzing data from 136,736 patients with DM and from matched cases from the National Health Insurance program in Taiwan between 2000 and 2015 (
21). Although Wang et al. found that the increased risk of incident NTM-caused diseases was observed in both patients with type 1 DM and those with T2DM, the risk of NTM disease development was not evident in our cohort composed of only T2DM patients. Though the reason for these disparate results is unclear, the severity of DM could be a possible explanation; the patients of the present study cohort were diagnosed with T2DM based on the ICD-10 (International Classification of Diseases, 10th Revision) code alone, irrespective of whether the patients received oral antidiabetes agents or insulin treatment.
There could be several plausible biologic explanations of the association of diabetes and risk for NTM disease. Pavlou et al. reported that long-term hyperglycemia impairs phagocytosis and bactericidal activity of macrophages, which is manifested as a reduced expression of interleukin-12 and diminished nitric oxide production (
25). In addition, patients with DM have strongly reduced gamma interferon production (
13), which is an essential cytokine for the eradication of the growth of NTM species (
26). Hyperglycemia increases vulnerability to infection by degrading neutrophil and T-cell function, decreasing humoral immunity, and downregulating the secretion of inflammatory cytokines (
27–29). Moreover, diabetic mice showed a diminished T-helper 1 adaptive immunity by lower expression of interferon and interleukin-12 and subsequent lower T-cell response to the
Mycobacterium tuberculosis antigen (
30). All of these findings strongly imply that DM directly suppresses the host innate and adaptive immune responses to NTM infection and increase susceptibility to NTM disease.
The present study assessed whether the development of NTM disease increased in accordance with the severity of diabetes by using DM-related complications as a proxy marker for DM severity similar to that in a previous study (
16). Notably, we found that the risk of NTM disease was evident only in patients with T2DM who had more diabetes-related complications. A number of reports have shown that immunity is further impaired when diabetes is uncontrolled. For instance, it was found that diabetic patients who had uncontrolled glucose levels were more likely to have impaired T-cell dysfunction (
31). In addition, Kumar et al. found that systemic levels of several proinflammatory cytokines showed a significant positive association with the glycated hemoglobin, which is a laboratory-evaluated parameter that directly reflects diabetes control (
32). Moreover, bone marrow-derived macrophages generated under the conditions of long-term hyperglycemia showed several intrinsic changes, including impaired phagocytosis and bactericidal activity (
25). As previous studies consistently reported that diabetes-related complications are significantly more prevalent in long-term uncontrolled diabetes than in controlled diabetes (
33,
34), our results on the association between the number of diabetes-related complications and the occurrence of NTM disease appear to be convincing and are in line with the findings of an earlier study which reported that tuberculosis risk in patients with DM increased significantly with an increase in the number of diabetes-related complications (
16).
Additionally, the duration of DM itself could be related to impaired immunity (
28,
35). To analyze the impact of duration of T2DM on NTM disease development, we reanalyzed our data by dividing the matched pair into two groups: (i) 76,151 patients diagnosed with T2DM before 2007 and their matched pair and (ii) 115,067 patients with new-onset T2DM from 2008 to July 2019 and their matched pair (
Fig. 1). The results of multivariable analyses in each group are shown in Tables S2 and 3 in the supplemental material and indicate that the relationship between the higher number of diabetes-related complications and the risk of NTM disease was evident only in those who were diagnosed with DM before 2007 (adjusted HR, 1.68; 95% CI, 1.18 to 2.38,
P = 0.004 [Table S2]). These findings supported the hypothesis that patients with longer-duration T2DM were more likely to have an impaired immune system, which increased susceptibility to infectious disease, including NTM disease.
Approximately two-thirds of patients in the present study did not receive treatment for DM, and the rate seemed to be comparable to that of a previous study of an analysis of data from a National Health Insurance Service (NHIS) claim database from 2002 and 2013, which revealed that 53.9% of patients with T2DM did not use any antidiabetes drug (
36). However, the proportion was lower than that in another study using data from the Korea National Health and Nutrition Examination Survey between 2016 and 2018, which reported that the proportion of DM patients receiving treatment with oral antidiabetes agents or insulin was approximately 60% (
9). It was found that the proportion of patients receiving treatment in South Korea tended to gradually increase over time during 2007 to 2017 (
37). Therefore, the low prescription rate in our study could be partially attributed to the fact that approximately 40% of our participants were diagnosed before 2007. In addition, it was possible that patients with IFG may have been misdiagnosed with DM, as a recent study in South Korea reported that the prevalence of IFG is nearly twice that of DM in the general population, which includes 27% of adults aged ≥30 years (
9).
The present study had several limitations. First, we determined NTM disease on the basis of ICD-10 codes alone instead of microbiological data. This was because the detailed results of NTM culture data were not available in the NHIS-National Sample Cohort (NSC) data (
38). Therefore, we were unable to confirm whether patients completely met the diagnostic criteria of NTM disease. In addition, the ICD-10 code of NTM disease alone cannot distinguish between pulmonary infection and other extrapulmonary disease. Therefore, we could not evaluate the burden of specific types of NTM disease in our study, although NTM pulmonary disease likely comprised the majority of the burden, as evinced in previous reports (
39–41). Third, the ICD-10 codes for comorbidities other than NTM infection may have been unreported if the attending physician did not include the diagnostic codes in the insurance claim. Thus, it was possible that comorbidity may have been underestimated in our study participants. Moreover, in our cohort, the proportion of patients with comorbid diseases was higher in the T2DM group than in matched cohorts. Considering that patients with a higher number of comorbid diseases were likely to have visited hospitals more often than those who had fewer comorbidities, it was possible that these frequent visits increased the likelihood of diagnosis of NTM diseases. Although we adjusted all comorbid diseases in the multivariate analysis, there exists a possibility of further NTM detection in T2DM patients with a higher number of comorbidities. Finally, because the data concerning human immunodeficiency virus (HIV) are masked in the NHIS-NSC database, as this diagnostic code constitutes sensitive personal information (
42), we were unable to adjust for HIV in our analysis. However, considering that the burden of HIV infection is low in South Korea (incident HIV infection rate in 2011 was 1.8 per 100,000 individuals [
43]) and the prevalence of NTM disease was only approximately 2% (19/1,060) in HIV-infected patients in Southeast Asia (
44), the cases of NTM disease associated with HIV infection would be low in our cohort.
In conclusion, this study involved an analysis of population-based cohort data in South Korea and showed an increased risk for NTM disease in T2DM patients with ≥2 diabetes-related complications. This finding suggested that T2DM patients with a larger number of complications should be regarded as a high-risk group for NTM disease.