Our data clarify the clinicopathologic and immunological features in patients with anti–IFN-γ Abs. We confirmed that the patients experienced recurrent infection, particularly NTM and skin dermatoses. An unspecific clinical presentation of primary DLBCL in the jejunum and NTM lymphadenitis without granuloma formation was observed. A limitation of the present study included the heterogeneous patient group, who were in various infectious statuses, such as active, inactive, or chronic. We were also unable to exclude the action of corticosteroid which may be prescribed for these patients on lymphocyte subset levels. However, we compared immunological data of 12 cases leaving antimicrobial therapy with 18 age-matched healthy individuals to diminish the effect of diverse clinical conditions on the data. The T cells were activated with polarization to Th1, Th17, and Treg. Lower ratios of RTE cells to naïve T cells might be indicate of lower thymic output in these patients. Although the number of NK cells was increased as in previous reports [1, 10], the percentages of subsets expressing activating markers were decreased. Upon stimulation, cytokine expression in T and NK cells was lower when compared with the control group.
IFN-γ plays an important role in granuloma formation for mycobacterial infection. Recurrent NTM infection is one of the most important clinicopathological features in patients with anti–IFN-γ Abs. For early diagnosis, histologic examination and acid-fast staining may be helpful. Microscopically, four histologic patterns have been detected [16]. That includes necrotizing and non-necrotizing granulomas, suppurative granulomas, and a histologic response lacking granulomas. In our study, 16.7% of patients with anti–IFN-γ Abs had suppurative necrosis with limited granuloma formation in lymph nodes. In addition, 22.2% of patients showed a mixture of large pale histiocytes, lymphocytes, plasma cells, and few granulocytes. No granulomas were detected. The effective immune response generated by IFN-γ in response to mycobacterial infection to form granulomas seems defected due to the presence of neutralizing anti-IFN-γ Abs, and it is necessary to maintain a high index suspicion of mycobacterial infection in such patients.
Administration of anti-CD20 monoclonal antibody (rituximab) has been reported to effectively reduce autoantibody titers, improve IFN-γ signaling, and achieve clinical remission [17–19]. Browne et al. was the first to demonstrate 4 patients treated with rituximab against refractory disseminated NTM [20]. After treatment with rituximab, the clearance of infection, resolution of inflammation, and reduction of autoantibody titers were improved. Furthermore, IFN-γ–induced STAT-1 phosphorylation was preserved. In the present study, the patients with rituximab therapy had more opportunities to discontinue antibiotic therapy. This treatment strategy seemed reasonable because of the unique mechanism by which rituximab attacks CD20 positive cells involved in antibody production. However, long-term follow-up data is limited. Despite rituximab administration, refractory and relapsed cases were also reported [7, 21]. Further research is necessary to evaluate the outcome of anti-CD20 therapy.
Current understanding of immune dysfunction in patients with anti–IFN-γ Abs is limited. In a study conducted by Browne et al., lymphocyte phenotyping showed a significantly decreased T cell percentage and naïve T lymphocytes, which was consistent with our results [1]. In addition, we found that the percentages of T cells with activation and polarization to Th1-like and Th17-like subsets were increased, based on dynamic analysis of T cell subpopulations. It raises the question whether decreased non-activated T cell subpopulations are age- or disease-related thymic impairments or just a dynamic fluctuation of T-cell subsets due to infection. To objectively evaluate the thymic function, the percentage of RTEs representative of the youngest T cells in the PBMCs were detected and analyzed with or without age adjustment. The ratio of RTE to naïve T subsets was compared between the disease and control groups to clarify thymic output during dynamic changes that occur during the immune response. Our data suggested that patients with anti-IFN-γ Abs would have lower levels of RTEs in the peripheral T-cell pool. This may impact the replenishment and diversity of T cells, and result in a delayed regeneration of T cells with a broad TCR repertoire [22].
Furthermore, an expansion of CD38 + HLA-DR + T cells was detected in the case group. High expressions of CD38 + HLA-DR + CD4 + and CD38 + HLA-DR + CD8 + T-cell subsets, indicative of T-cell activation, are well-known in viral and bacterial infections [23–25]. However, repetitive or prolonged antigen stimulation may induce expression of exhaustion markers and decrease IFN-γ production [26]. Wang et al. demonstrated patients with fatal H7N9 outcomes displayed higher and prolonged expression of CD38 + HLA-DR + on CD8 + T cells than those who survived [24]. The majority of these activated T cells concurrently expressed high levels of PD-1 with minimal IFN-γ production. In addition, a delay of clonally expanded TCRαβ clonotypes within CD38 + HLA-DR + CD8 + T cells were featured in fatal cases. Similarly, our data revealed higher percentages of CD38 + HLA-DR + CD4 + and CD38 + HLA-DR + CD8 + T cells in the patients with anti-IFN-γ Abs. These patients suffered from adult-onset immunodeficiency due to defects in IFN-γ immune surveillance that may lead to recurrent infection. While the status of PD-1 expression was not examined, the decreased expression of intracellular cytokines in the disease group supported exhausted function in these T cells.
In agreement with previous studies, the number of NK cells was increased in patients with anti-IFN-γ Abs [1, 10]. Chronic and repeating opportunistic infections may be the predominant cause for the increase of NK cells. We further evaluated subpopulations and functional markers of NK cells. Two major NK subpopulations, CD56bright and CD56 + CD16 + subsets were decreased. Expression of CD57 was increased when comparing the patients with the healthy subjects. Moreover, the NK cells diminished the expression of NKp30 and NKp46 and the production of TNF-α and IFN-γ. Indeed, CD56 + CD16 + NK cells with a high cytolytic potential differentiate from CD56bright NK cells, which proliferate and produce IFN-γ in response to stimulation [27, 28]. CD57 cells define a subset of highly mature cells in the NK population [29]. The expression of activation receptors, including NKG2D, DAP10, CD16, natural cytotoxicity receptors (NKp30, NKp44, and NKp46), CD226, and 2B4 are downregulated during NK-cell maturation [30]. NK cells exhibiting impaired effector functions have also been reported in hosts with tumors or chronic infections [31]. NK cells with exhausted effector functions show decreased cytokine production and cytolytic activity. Altogether, theses data indicate the exhausted immune status of NK cells in patients with anti-IFN-γ Abs. Further study is necessary to clarify the functional exhaustion in the expanded CD57 + subpopulation.
The study of the ability of lymphocyte subpopulations to produce cytokines in patients with anti-IFN-γ Abs is limited. While engulfing mycobacteria, mononuclear phagocytes secret IL-12 to activate T- and NK-cells and introduce IFN-γ production. The IFN-γ biding signals enable macrophage activation with upregulation of expressions of TNF-α and IL-12 through phosphorylated STAT1- and NFκB essential modulator-mediated pathways [2]. TNF-α and IFN-γ are essential for granuloma formation, which is hallmark of a mycobacterial infection. Anti-IFN-γ Abs may block production of downstream mediators of IFN-γ activity. Krisnawati et al. reported a significant decrease in levels of serum TNF-α and IFN-γ in all tested sera of patients with anti-IFN-γ Abs [9]. Production of IL-2 and TNF-α in CD4 + cells and TNF-α in CD8 + cells was significantly lower in a study conducted by Wipasa et al. [11]. The present study found similar results, with a decrease in cytokine production in lymphocyte subpopulations. That may explain the lack of formation of granulomas in some patients, although the cytokine production varied a great deal. On the other hand, the reduction of cytokine responses may account for immune exhaustion and lead to a higher risk of disseminated NTM or other opportunistic infections, like patient 19 who was infected by L. monocytogenes.