The development of active tuberculosis (TB) is increased in immunocompromised patients and is closely related to determinants for disease reactivation or newly acquired TB infection. A targeted detection of latent tuberculosis (LTBI) in these high-risk groups should be performed especially if preventive treatment is planned. Despite the better diagnostic accuracy of interferon-gamma immuno assays (IGRAs) over the TST, the performance of immunodiagnostic tests is highly variable among different groups of immunocompromised people.
Clinical investigations about the presence and extent of antigen-specific T-cell immunity are becoming more important and the need for antigen-specific functional T-cell assays that can be performed in routine diagnostic clinical laboratories is wideranging and significant.
A strategy incorporating different immunological assays and the analysis of TB epidemiological and clinical risk factors, may be capable of generating novel immune profiles to improve LTBI detection at screening and to identify those at risk of developing TB.
Assessment of reactivation risk for patients with LTBI remains challenging. Approximately 90% of immunocompetent individuals with LTBI will never develop active TB. In fact, 80% of human lung calcified granulomas do not contain viable Mtb, suggesting that most immunocompetent individuals can either clear or effectively contain infection. Therefore, new biomarkers to reliably identify the 10% of immunocompetent patients with LTBI with reactivation potential are highly desirable, in particular for patients developing immunosuppression with the highest risk of TB reactivation.
The detection of LTBI in compromised patients is closely dependent on determinants for disease reactivation or newly acquired TB infection. Such factors include exposure and/or endemicity of Mtb infection, the degree and type of mmunosuppression, the presence of comorbid illness and concomitant medical treatment. Despite the better diagnostic accuracy of IGRAs over the TST, the performance of immunodiagnostic tests is highly variable among different groups of immunocompromised people.
In the last few years several studies have described the role of CD8+ T-cell responses in TB [Brookes RH. 2003]. Mtb-specific CD8+ T cells have been associated with active TB, both in HIV uninfected and infected patients, and to recent infections in adults and young children recently exposed to a smear-positive active TB case [Rozot V. 2015; Day CL. 2011; Lancioni C. 2012]. Infact recent contacts of active TB patients, have a greater CD8+ T-cell response compared to other study groups as active TB patients, healthcare workers and BCG-vaccinated healthy controls) [Nikolova M. 2013]. This is in agreement with findings observed in a cattle model where a CD8+ T-cell response is present at the onset of infection [Ryan AA. 2009]. The IFN¿ release assays currently in use primarily elicit a CD4+ response, but emerging data provide a good rationale for measuring specific CD8+ T-cell responses, and in particular to further investigate the association between CD8+ T cells and risk of disease progression. The introduction of new peptides eliciting a CD8+ T-cell response is expected to increase the sensitivity for LTBI detection, infact, CD8+ T-cell work at the early phase of Mtb infection and at a phase of reactivation from LTBI [Rozot V. 2015; Barcellini L. 2016].
Detection of previous exposure to a pathogen is an important aspect of clinical investigation. Traditionally, this has been accomplished via two strategies. First, by detection of the pathogen in the subject, this can be carried out by culturing of patient fluid, or more recently by molecular techniques such as quantitative polymerase chain reaction.
The second approach has been to analyze components of the immune system for their ability to specifically recognize the pathogen in question, this has most commonly been achieved in the clinical environment by serology testing, but an third approach could include the T-cell function detection methods, for pathogens such as tuberculosis. This last approach has the advantage of not only informing the clinician whether the patient has previously been exposed to the pathogen, but also can indicate the degree of specific immune response the patient is capable of mounting. The potential use for this assay in assessing T-cell functional responses in routine clinical laboratories is wideranging and significant.
Clinical investigations on the presence and extent of antigen-specific T-cell immunity are becoming more important and the need for antigen-specific functional T-cell assays that can be performed in routine diagnostic clinical laboratories is wideranging and significant.
With a view to improving the identification of LTBI cases among high-risk compromised patients, we would like to evaluate the combination of immunological markers to measure a broad polyclonal response to Mtb-specific antigens, providing a simple assay of T lymphocyte function, comprising IFN¿ secretion, T cell proliferation, surface co-expression of CD25/CD134 and multifunctional CD4+ and CD8+ T-cell responses.
In conclusion, the detection of LTBI in different groups of high-risk immunocompromised patients remains a challenge. The development of new immunological biomarkers and integrated clinical and epidemiological strategies represents a great advance to identify LTBI in individuals at risk of developing active TB, and to plan preventive chemotherapies.