Antigen-specific intraocular cytokine responses distinguish ocular TB from undifferentiated uveitis

Together with appropriate clinical signs, these criteria include evidence of current or previous TB infection (histologically or microbiologically proven TB, or positive interferon gamma release assay [IGRA] or positive tuberculin skin test [TST]), and negative tests for syphilis and sarcoidosis. Histological and/or microbiological evidence of TB are rarely found in ocular samples. Hence the tests for immunoreactivity to TB antigens (TST and/or IGRA) become critical to the etiological diagnosis of TB, provided the other conditions are fulfilled. Additionally, given the limitations of currently available molecular diagnostic tests (such as polymerase chain reaction [PCR]), much reliance is placed on the TST/IGRA for diagnosis.

TB immunoreactivity denotes the memory T-cell response (antigen recall) to TB antigens that can be tested in vivo (as in TST) or in vitro (as in IGRA). Such T cell memory responses can be detected during active disease, latent infection, or even after the infection has been cleared from the body. The inability of TST or IGRA to distinguish active TB disease from inactive TB infection represents a formidable challenge in the diagnosis of OTB. This has therapeutic implications since it may lead to overdiagnosis of OTB, and initiation of potentially toxic anti-TB therapy (ATT), especially if appropriate clinical signs are absent and non-TB diagnoses are not ruled out. Conversely, especially in TB-endemic countries, the significance of a positive IGRA or TST may be downplayed by the treating ophthalmologist, thus denying ATT to the patient, and prolonging intraocular inflammation.

There exists a large subset of patients with undifferentiated uveitis and positive TB immunoreactivity, in both TB-endemic and non-endemic countries. The current classification criteria do not recognize these patients as OTB, 3 and the need for initiating ATT in such patients remains unclear. Furthermore, the impact of systemic TB immunoreactivity on the intraocular immune response of this large cohort of uveitis patients remains largely unknown.

In the current study, Kaiser Alam , Gunjan Sharma , John V. Forreste et al investigated the intraocular antigen-specific immune response in different clinical phenotypes of TB-immunoreactive uveitis. They have applied the SUN classification criteria for OTB, and/or positive TB PCR to strictly differentiate 'true' OTB from TB immunoreactive uveitis of unknown origin (labelled as UNK group). Their study revealed a clear separation between these two subgroups in the cytokine response to TB-specific antigen.

It was a single center, retrospective cross-sectional study including patients requiring diagnostic or therapeutic vitrectomy for the management of intraocular inflammation, were divided into three groups based on Standardization of Uveitis Nomenclature (SUN) classification criteria for tubercular uveitis.

(1)Ocular tuberculosis (OTB, n=23): TST+ patients, meeting the SUN criteria, and/or polymerase chain reaction (PCR)+ for TB.

(2) Uveitis of unknown origin (UNK, n=24): undifferentiated TST+ patients not meeting SUN criteria.

(3) Non-TB uveitis patients (n=24): TST- patients, with or without well-defined non-TB diagnosis.

Total vitreous cells were activated with Mycobacterium tuberculosis–specific Early Secreted Antigenic Target-6 (ESAT-6) or the retinal autoantigen, interphotoreceptor retinoid binding protein peptide (pIRBP 1-20), stained for intracellular IFN-γ, TNF-α, and IL-17, and analysed by flow cytometry. Antigen-specific single and dual (polyfunctional) cytokine responses to ESAT-6 and IRBP, were compared between the three groups.

All cytokine responses to ESAT-6 were higher in the UNK group compared to non-TB controls, while all except IL-17, were comparable between the OTB and non-TB groups. Polyfunctional responses – IFNg-IL-17 (p=0.002), TNFa-IL-17 (p=0.02) and TNFa-IFNg(p=0.01), were significantly greater for UNK than the OTB group. Polyfunctional cells also produced more cytokine per cell than respective monofunctional cells. IRBP cytokine responses were comparable between different groups, and not affected by the clinical phenotype, or duration of disease.

Tests for TB immunoreactivity (TST and IGRA) are central to the current diagnostic strategy for OTB. Their role in OTB diagnosis has been reinforced by the publication of the machine learning based SUN classification criteria. However, the utility of these tests is undermined by the high prevalence of TB immunoreactivity in the general uveitis population with a majority of such patients being classified as uveitis of unknown origin (UNK).

This study revealed fundamental differences in the intraocular immune response between undifferentiated TST+ patients with uveitis (UNK group) and patients classified as OTB based on the SUN criteria, and/or positive TB-PCR.

Authors noted that the cytokine responses to M. tuberculosis antigen ESAT-6 were significantly higher in the UNK group not only compared to non-TB uveitis patients but also to the OTB group. Specifically, the UNK and OTB groups differed in the polyfunctional (dual cytokine) responses to ESAT-6, and the polyfunctional cells expressed higher levels of pro-inflammatory cytokines than the corresponding monofunctional cells. In addition, they also demonstrated that retinal autoantigen IRBP specific intraocular cytokine responses occurred in all cases of posterior segment uveitis, regardless of their clinical phenotype, chronicity, or TB immunoreactive status.

Study data offers mechanistic support to the SUN classification criteria for tubercular uveitis. They provide a concrete immunological basis for future studies on diagnostic tests, biomarkers and treatment outcomes in OTB, to follow the SUN criteria for patient inclusion. For TBimmunoreactive (TST+) patients, who cannot be classified as OTB based on the SUN criteria (UNK group in the current study), the data suggest that judicious anti-inflammatory therapy without the need for adjunctive ATT may be sufficient to control inflammation and prevent immune-mediated tissue damage.

This study also documented the antigen-specific intraocular response to the retinal antigen, IRBP, a known uveitogenic autoantigen in experimental models. Immune responses to retinal antigens have been observed in the peripheral blood in uveitis patients but may also be present in normal individuals.

"In summary, our study demonstrates that TST+ undifferentiated uveitis generates a stronger monofunctional and polyfunctional intraocular cytokine response than active OTB, suggesting that the anti-TB immune response in TST+ undifferentiated uveitis is more effective in protecting from pathogen-based tissue damage. An additional autoreactive anti-IRBP response was characteristic of both TB- and non-TB- associated intraocular inflammation."

Source: Kaiser Alam , Gunjan Sharma , John V. Forrester , Soumyava Basu , Antigen-specific intraocular cytokine responses distinguish ocular TB from undifferentiated uveitis in TB-immunoreactive patients, American Journal of Ophthalmology (2022), doi: https://doi.org/10.1016/j.ajo.2022.08.029