The exhaustion of lymphocytes is the main factor that decreases the sensitivity of QFT-GIT detection in silicosis

Tuberculosis is a common and serious complication of silicosis. The probability of concurrent pulmonary tuberculosis at the time of phase I–II disease is 10–30%. When silicosis enters phase III, tuberculosis occurs in 50–90% of patients, and 45% of silicosis patients die directly from tuberculosis [1‐3]. Studies have found that with the development of silicosis, the incidence of pulmonary tuberculosis increases [2]. At the same time, when silicosis is complicated by pulmonary tuberculosis, the two diseases promote each other and accelerate deterioration, so timely detection of tuberculosis infection and targeted treatment are particularly important for patients with silicosis [2, 3].

The IFN-γ release in vitro test (TB-IGRA) is a novel detection method for the early diagnosis of clinical pulmonary tuberculosis. It is an in vitro diagnostic method that tests the release of IFN-γ to help diagnose tuberculosis or determine whether the body is infected with mycobacterium tuberculosis. The principle is that after mycobacterium tuberculosis infection, the body can produce antigen-specific T cells, and when the body is stimulated by mycobacterium tuberculosis antigen again, the antigen-specific T cells can be quickly activated and proliferate, releasing IFN-γ and thereby aiding in the diagnosis of tuberculosis [4]. Compared with traditional detection methods, TB-IGRA has several positive characteristics, such as rapidity, high sensitivity and high specificity, so it is accepted by most of laboratories. Since TB-IGRA is a detection method based on the host's immune response to mycobacterium tuberculosis, the results of TB-IGRA are closely related to the patient's immune status [4]. Existing studies have confirmed that compared with other tests (TST, X-ray and culture), TB-IGRA is considered to be the most sensitive method of tuberculosis diagnostics [5]. Of the two commercially available TB-IGRA, QuantiFERON-TB Gold (QFT-GIT) assays are more commonly used in routine clinical practice than T-SPOT.TB assays. Some research has compared the performance between TST and TB-IGRA in silicosis complicated by tuberculosis and found that TB-IGRA is more suitable for tuberculosis diagnostics. However, there has been no study on whether silicosis can affect the sensitivity of TB-IGRA [6, 7]. Therefore, the study systematically analyzed the effect of silicosis on QFT-GIT assays and explored the causes of and solutions.

Silicosis is the most common type of pneumoconiosis [3]. It is caused by long-term inhalation of large amounts of free silica dust and is mainly characterized by extensive pulmonary nodular fibrosis. When severe, it affects respiratory function, and the patient loses the ability to work. Because silica damages phagocytic function, affects peripheral cells, and interferes with cytokine production, silica affects immune function and can easily predispose patients to tuberculosis [2, 8, 9]. The CT test of pneumoconiosis combined with pulmonary tuberculosis are more complicated, lacking typical clinical manifestations and being easily missed in clinical practice [10]. Existing studies have confirmed that TB-IGRA is the most sensitive test for the detection of patients with silicosis combined with pulmonary tuberculosis [11]. It can not only detect active tuberculosis but also prompt latent tuberculosis infection (LTBI) to provide an important basis for clinical treatment [5‐7]. Early studies have confirmed that the false-negative rate of patients with low immunity (HIV infection) will increase significantly and other study also found that patients with silicosis have immunodeficiency [12, 13]. However, there is no study on whether this immunocompromised state will affect the sensitivity of TB-IGRA. Therefore, further analysis of the relationship between immune function and TB-IGRA will provide clinically more accurate test reports, which is conducive to more accurate diagnoses of tuberculosis.

The study found that the diagnostic efficiency of QFT-GIT in the patients from the TB&SILs group was significantly reduced but the proportion of CD4⁺T cells, CD8⁺T cells and CD56⁺NK in the TB&SILs group were no difference with TB group, SILs group and SILs&LTBI group.

Existing studies have found that PD-1/PD-L1 can inhibit the function of T cells [14, 15]. In the study of tumours and viruses, it has been found that high concentrations of antigens for a long time can induce CD4⁺T cells to express PD-1, decreases the IFN-γ expression of lymphocytes and lead to immune exhaustion [16, 17]. Some studies also found that silica instillation significantly elevated the expression of immune checkpoint molecules such as PD-1 in mice and suggested that silica is an important risk factor for T-cellcell exhaustion [18, 19]. Due to the negative correlation between PD-1 and IFN-γ and there is no study on T cell exhaustion in silicosis patients before, this study analysed the expression of PD-1 in silicosis and found that the peripheral blood lymphocytes of patients with silicosis highly expressed PD-1, and the PD-1 highly expression can inhibited the secretion of IFN-γ by lymphocytes (especially CD4⁺T cells). Further analysis found that the ratio of PD-1⁺CD4⁺T and IFN-γwere negatively correlated and blockaded the PD-1 pathway with antibodies can restore the sensitivity of QFT-GIT in silicosis.

Tuberculosis is the main cause of death in silicosis, and many studies have found that cytokines (IFN-γ, IL-1β and TNF-α) that can inhibit the immune escape of tuberculosis are decreased, but some Th2 cytokines are increased in silicosis [20, 21]. The main reason for the high incidence of tuberculosis in patients with silicosis is still unclear, and the methods for early diagnosis of pulmonary tuberculosis in silicosis patients are very limited. Research has indicated that higher expression of PD-1 inhibits the Th1 immune response, which suppresses the anti-TB immune response. This study found that the peripheral blood lymphocytes of patients with silicosis highly expressed PD-1, and the immune depletion of CD4⁺T cells and NK cells was the main reason for the higher false negative rate of QFT. The findings of this study suggest that patients with silicosis have immune exhaustion, which provides a new theory for the occurrence of silicosis combined with pulmonary tuberculosis. However, this study is not sufficiently systematic and in depth, and we analyzed the molecular mechanism of silicosis leading to a high incidence of tuberculosis and used animal models to explore the role of PD-1 in silicosis, which will provide new targets for the treatment and immune monitoring of silicosis.

This is the first study to analyze the relationship between immune exhaustion and QFT-GIT in silicosis and found that the sensitivity of QFT-GIT was decreased by the expression of PD-1 on lymphocytes. Antibody blocking experiments increased the expression of IFN-γ and provided a new method to improve the sensitivity of QFT in silicosis. The study also found that silicosis can increase PD-1 expression, as PD-1 functions in infectious diseases, prompting immune exhaustion in silicosis, which may promote tuberculosis from latent to active infection. The study provided theoretical evidence for the diagnosis and immunotherapy of silicosis complications, and it has great value in clinical diagnostics and treatment.