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To the Editor—Tham et al report 3 patients with nontuberculous mycobacteria (NTM) infections and varying levels of neutralizing anti-interferon-gamma (IFNγ) autoantibodies, highlighting the complex and nonlinear relationship between autoantibody levels and clinical disease . These relationships are quite similar to those we reported previously . It is important to reiterate that the majority of cases of disseminated NTM with anti-IFNγ autoantibodies do not require treatment with rituximab. However, for those with treatment refractory disease who have persistently high anti-IFNγ neutralizing immunoglobulin G levels, rituximab has been effective [2, 3]. In those settings, the fall and rise of intra-individual titers appear to be good correlates for therapeutic response .
Important factors that affect the biologic impact of anti-IFNγ autoantibodies include the antibody type and subclass, the targeted epitope and the binding avidity. In addition, we fully agree that cofactors extrinsic to anti-IFNγ autoantibodies likely influence host susceptibility, such as virulence of the infecting organisms, as well as the patient's background genetics, nutrition, smoking status, and general health. However, what virtually all the cases of anti-IFNγ autoantibodies have in common, including those of Tham et al, is precisely the cytokine neutralization that leads to a phenocopy of genetic defects in the IFNγ/interleukin 12 pathway, suggesting that this is a unifying pathogenic mechanism.
Our patient had disseminated, drug-sensitive tuberculosis, associated with anti-IFNγ autoantibodies, yet still developed a severe paradoxical immune reaction . This suggests that intact IFNγ signaling is dispensable for the tuberculosis paradoxical reaction. Similar observations have been made with hemophagocytic lymphohistiocytosis in patients with defects in the IFNγ receptor .
In summary, we agree with Tham et al that there are likely to be unappreciated cofactors, both host and microbial, which determine what level of IFNγ neutralization is required to permit severe infection in any given individual. Those factors and permissive antibody titers vary from person to person, but once it has occurred, the titer is a reasonable longitudinal marker of neutralization severity .
Potential conflicts of interest.All authors: No reported conflicts. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.