In this study, we confirmed that anti-IFN-γ autoAbs are an important risk factor for T. marneffei infection among non-HIV-infected patients. Most patients who were anti-IFN-γ autoAb-positive did not have any underlying respiratory disease and frequently had systemic dissemination with major pleural effusion. In addition, the leukocyte count and the levels of C-reactive protein and other inflammatory markers in these patients were significantly higher than those in anti-IFN-γ autoAb-negative patients. Despite the progression of T. marneffei infection, the anti-IFN-γ autoAb titer did not decrease after targeted treatment, and most patients had a poor outcome (such as death or recurrence of infection).
Anti-IFN-γ autoAbs have previously been shown to be an important risk factor for NTM and T. marneffei infections. Our data also supported this important finding. A large number of anti-IFN-γ autoAb-positive patients were found in both group 1 and group 2. This might be related to the high prevalence of anti-IFN-γ autoAb-associated HLA class II DRB1*16:02 and DQB1*05:02 alleles in the Asian population[20, 21]. Both Guangdong and Guangxi provinces are located in southern China, where NTM and T. marneffei are endemic due to the humid climate. In group 3, only two patients were anti-IFN-γ autoAb-positive, possibly because cryptococcal infection is associated with other anti-cytokine antibodies. A recent study also confirmed the link between the presence of neutralizing anti-granulocyte macrophage colony stimulating factor autoAbs and cerebral cryptococcosis, particularly that caused by C. gattii infection.
Since the 1990s, an increasing number of patients with T. marneffei infection have been reported among non-HIV-infected patients with impaired cell-mediated immunity. The comorbidities included primary adult-onset immunodeficiency due to anti-IFN-γ autoAbs and secondary immunocompromise, including that resulting from autoimmune disease or the use of immunosuppressive drugs such as novel anticancer targeted therapies and kinase inhibitors, but infections have been found even in patients with normal immunity. The pathogenesis of anti-IFN-γ autoAb-positive patients was different from that of anti-IFN-γ autoAb-negative patients who had normal immunity and suffered from chronic lung diseases (such as COPD, bronchiectasis or asthma). Chronic lung diseases can lead to lung structural damage in various ways, resulting in impaired natural immune function and thereby changing the microenvironment that offers a niche for respiratory microorganisms. Furthermore, dysmicrobiosis might have increased the susceptibility of the hosts to T. marneffei infection[24, 25]. As a result, the lungs were the dominant organs involved, whereas anti-IFN-γ autoAb-positive patients were more likely to develop systemic dissemination of T. marneffei infection and pleural effusion. Most patients with T. marneffei infection had pleural effusion characterized by yellowish exudates, with marked elevation of protein levels and nucleated cell counts. Multiple organs might also be involved, especially the bone and skin, which might readily predispose patients to the development of Sweet syndrome.
Patients who tested positive for anti-IFN-γ autoAbs also had more significantly elevated inflammatory responses characterized by elevated leukocyte and neutrophil counts, erythrocyte sedimentation rates and C-reactive protein levels, markers indicating more exuberant infections in patients with anti-IFN-γ autoAbs. Neutrophils play an important role in the development of innate immunity. Neutrophils are the frontline barrier for eradicating the invasion of microbial pathogens and have powerful phagocytic capacity. In addition, neutrophils have been implicated in the production of the chemokine myeloperoxidase. Neutrophil-derived IL-17A also induces the release of IFN-γ, which promotes the antibacterial activity of macrophages. We also observed a notable bone marrow response in this patient population, which was more prone to developing anemia and leukocytosis. A significant increase in CD8+ T cells was observed in anti-IFN-γ autoAb-positive patients. IFN-γ is indispensable for fighting infections because of its ability to regulate various protective functions and sustain the activity of both CD4+ and CD8+ T cells. Conversely, IFN-γ produced by CD8+ T cells promotes the expression of major histocompatibility complex molecules on the surface of target cells to enhance antigen presentation and enhances the ability of macrophages and dendritic cells to phagocytose pathogens. We speculate that the autoAbs might have affected the normal function of IFN-γ, leading to compensatory proliferation of these immune cells.
The patients were prospectively followed for up to 12 months, thus allowing us to estimate the correlation between the autoAb titer and disease progression. However, the anti-IFN-γ autoAb level did not correlate significantly with the clinical course, which was consistent with the findings from a previous study. In addition, anti-IFN-γ autoAb-positive patients were more likely to be coinfected with other intracellular pathogens, especially NTM[34, 35]. Despite the initiation of standardized antimicrobial therapy, the patients’ conditions did not improve, and some deteriorated even further. There is no well-established standard therapy to reduce the titer of anti-IFN-γ autoAbs. However, one study reported a significant improvement in clinical symptoms after plasma exchange therapy. Supplementation of IFN-γ recombinant protein and B cell depletion with an anti-CD20 antibody[37, 38] have also been successfully used as adjuvant therapies in combination with antimicrobial therapy in a small number of patients. Prospective randomized clinical trials are needed to determine the therapeutic efficiency of these strategies.
Some limitations of our study need to be acknowledged. First, our sample size was relatively small, and therefore, selection bias might have affected the interpretation of our data. Second, for some patients, documentation of the laboratory findings during the longitudinal follow-up was incomplete, which hampered assessment of associations with the therapeutic response. Moreover, this was an observational cohort study with a limited duration of follow-up, and additional immunological experiments are needed to further explore the underlying mechanism.