Kinetics of the immune response associated with tularemia: comparison of an enzyme-linked immunosorbent assay, a tube agglutination test, and a novel whole-blood lymphocyte stimulation test

Thoracic manifestations of tularaemia: a case series

BACKGROUND

Tularaemia is a zoonotic disease caused by Francisella tularensis, a highly virulent bacterium that affects humans and small wild animals. It is transmitted through direct contact with infected animals or indirectly through contaminated soil, water or arthropod bites (e.g. ticks). Primary thoracic manifestations of tularaemia are infrequent and, therefore, a diagnostic challenge for clinicians.

METHODS

We report six tularaemia cases with exclusively thoracic involvement diagnosed in a clinic for pulmonary diseases in Bavaria between 10/2020 and 02/2022.

RESULTS

All patients lived or were active in rural areas, four reported a recent tick bite. All patients presented with thoracic lymphadenopathy and pulmonary tumours or consolidations; all underwent bronchoscopy with EBUS-TBNA of lymph nodes, three lung biopsies as well. Five patients showed inflammatory changes in the endobronchial mucosa. The main histological findings were necrotic epithelioid granulomas with remarkable granulocyte infiltration. All cases were identified by positive serology, five by PCR (here identification of F.t. ssp. Holarctica) from biopsy as well. As first-line therapy, oral ciprofloxacin was given (5/6); in 2/6 cases, a combination of quinolone-rifampicin was given.

CONCLUSIONS

Pulmonary tularaemia may occur after tick bites and without extrathoracic manifestations. In patients who present with thoracic lymphadenopathy and pulmonary consolidations and who are exposed to increased outdoor activities, tularaemia should be included in the diagnostic pathway. Histologically, the presence of neutrophil-granulocyte infiltrations might help to distinguish tularaemia from other granulomatous infections, e.g. tuberculosis. The combination of quinolone-rifampicin rather than i.v. gentamicin reduced length of hospital stay in two patients.

Evaluation of In-House and Commercial Serological Tests for Diagnosis of Human Tularemia

Tularemia is a zoonosis caused by the bacterium Francisella tularensis Its specific diagnosis remains based on serological methods, while F. tularensis is rarely detected in clinical samples by culture or PCR. The aim of the present study was to evaluate the performance of the Serion enzyme-linked immunosorbent assay (ELISA) classic Francisella tularensis IgG and IgM tests (Virion/Serion GmbH Institute, Würzburg, Germany) and the VIRapid tularemia immunochromatographic test (ICT) (Vircell, Granada, Spain) compared to that of the in-house microagglutination test (MAT) and indirect immunofluorescence assay (IFA) currently used at the French National Reference Center for Francisella We evaluated 256 consecutive sera from 208 patients, including 51 confirmed and 23 probable tularemia cases, and 134 control patients not infected with F. tularensis The IFA tests displayed 72.5% sensitivity for IgM (cutoff titer ≥80) and 74.5% for IgG (cutoff titer ≥160), and 99.3% specificity for both IgM and IgG. Using cutoffs advocated by the manufacturer, the Serion ELISAs displayed 88.2% sensitivity for IgM and 86.3% for IgG antibodies; specificity was 94.8% for IgM and 95.5% for IgG. Compared to MAT and IFA tests, the Serion ELISAs allowed earlier detection of specific antibodies (1 to 2 weeks versus 2 to 3 weeks after the onset of symptoms). The ICT sensitivity and specificity were 90% and 83.6%, respectively, when considering the cutoff advocated by the manufacturer. In conclusion, the Serion ELISAs are useful as screening tests for tularemia diagnosis, but additional confirmatory tests (such as MAT and IFA) are needed, especially in areas of low endemicity.

Cat-bite-induced Francisella tularensis infection with a false-positive serological reaction for Bartonella quintana

Introduction. Tularaemia is caused by infection with Francisella tularensistransmitted via direct contact with an infected hare carcass or indirectly through the bites of vectors, but may be cat-bite-associated as well. Medical history and reliable diagnostic analysis are important in order to differentiate it from other cat-associated infections, e.g. Bartonella spp.

Casepresentation. A healthy 56-year-old man was examined because of a cat-bite-associated ulceroglandular wound on his right thumb. Nineteen days after the cat bite occurred, a serology test was positive for anti-Bartonella quintana, but negative for anti-F. tularensis. Since Bartonella infections are rare in Sweden, another serology test was analysed 2 weeks later with a positive result for anti-F. tularensis. The patient was treated with doxycycline for 14 days and recovered. The patient was re-sampled after 18 months to obtain a convalescent sample. The acute and the convalescent samples were both analysed at a reference centre, with negative results for anti-Bartonella spp. this time.

Conclusion. This case is enlightening about the importance of extending the medical history and re-sampling the patient for antibody detection when the clinical suspicion of cat-bite-associated tularaemia is high. The false-positive result for anti-B. quintana antibodies may have been due to technical issues with the assay, cross-reactivity or both.

Development and evaluation of a latex agglutination test for the rapid serodiagnosis of tularemia

A latex agglutination test (LAT) was developed for a rapid detection of antibodies against Francisella tularensis. The assay is performed by mixing serum with antigen-coated latex beads and read within 5 min. Developed LAT has been proved to be a specific, sensitive, fast, easy-to-perform and cost-efficient tool for the routine diagnosis of tularemia.

Immunological markers that correlate with protection immunity against tularemia infection

An efficient immune response to tularemia is dependent on a strong cell-mediated component. We tried to identify markers of cellular immune responses that indicate a vaccine efficacy against tularemia. BALB/c mice were immunized with mutant F. tularensis 15∆23A and/or F. tularensis 15 NIIEG strains and then were challenged i.n. with F. tularensis Schu. We compared the influence of F. tularensis antigens (tularinum) in vitro on production of IL-1, IL-5, IL-6, IL-17, IFN-γ, and TNF-α by splenocytes obtained from intact mice and mice immunized with mutant F. tularensis 15∆23A and/or F. tularensis 15 NIIEG strains. We also compared expression of CD28, CD154, TLR-2, and CD69 markers on CD4 and CD8 T-cells after activation with tularinum in vitro. We found that tularinum-induced CD4(+) T-cells increased TNF-α and IFN-γ synthesis and expression of CD69 only in group mice with high degree of post immunization protection against F. tularensis Schu challenge. Estimation of CD69 expression on CD3(+)CD4(+) cells and IFN-γ, TNF-α synthesis by CD4(+) T-lymphocytes could be useful for determination protect ability of antitularemia immunity.

Detection of Francisella tularensis-specific antibodies in patients with tularemia by a novel competitive enzyme-linked immunosorbent assay

A novel competitive enzyme-linked immunosorbent assay (cELISA) was developed and evaluated for detection of antibodies against Francisella tularensis in humans. The assay is based on the ability of serum antibodies to inhibit the binding of monoclonal antibodies (MAbs) directed against F. tularensis lipopolysaccharide antigens. The assay was evaluated using serum samples of tularemia patients, inactivated F. tularensis-immunized rabbits, and F. tularensis-infected mice. Antibodies against F. tularensis were successfully detected in serum samples of tularemia patients as well as the immunized and infected animals. The cELISA method was compared to indirect ELISA (iELISA) and the commonly used microagglutination test (MA) using serum samples of 19 tularemia patients and 50 healthy individuals. The sensitivity and specificity of cELISA were 93.9 and 96.1%, respectively, in comparison to the iELISA. MA was less sensitive than cELISA with a sensitivity and specificity of only 81.8 and 98.0%, respectively. A high degree of correlation (R(2) = 0.8226) was observed between cELISA and iELISA results. The novel cELISA developed in this study appears to be highly sensitive and specific for serodiagnosis of human tularemia. The potential of the MAb-based cELISA to be used in both human and animal samples emphasizes its usefulness for serological survey of tularemia among multiple animal species.

Immunity to Francisella

In recent years, studies on the intracellular pathogen Francisella tularensis have greatly intensified, generating a wealth of new information on the interaction of this organism with the immune system. Here we review the basic elements of the innate and adaptive immune responses that contribute to protective immunity against Francisella species, with special emphasis on new data that has emerged in the last 5 years. Most studies have utilized the mouse model of infection, although there has been an expansion of work on human cells and other new animal models. In mice, basic immune parameters that operate in defense against other intracellular pathogen infections, such as interferon gamma, TNF-α, and reactive nitrogen intermediates, are central for control of Francisella infection. However, new important immune mediators have been revealed, including IL-17A, Toll-like receptor 2, and the inflammasome. Further, a variety of cell types in addition to macrophages are now recognized to support Francisella growth, including epithelial cells and dendritic cells. CD4⁺ and CD8⁺ T cells are clearly important for control of primary infection and vaccine-induced protection, but new T cell subpopulations and the mechanisms employed by T cells are only beginning to be defined. A significant role for B cells and specific antibodies has been established, although their contribution varies greatly between bacterial strains of lower and higher virulence. Overall, recent data profile a pathogen that is adept at subverting host immune responses, but susceptible to many elements of the immune system's antimicrobial arsenal.

Evaluation of an immunochromatographic test for rapid and reliable serodiagnosis of human tularemia and detection of Francisella tularensis-specific antibodies in sera from different mammalian species

Tularemia is a highly contagious infectious zoonosis caused by the bacterial agent Francisella tularensis. Serology is still considered to be a cornerstone in tularemia diagnosis due to the low sensitivity of bacterial culture and the lack of standardization in PCR methodology for the direct identification of the pathogen. We developed a novel immunochromatographic test (ICT) to efficiently detect F. tularensis-specific antibodies in sera from humans and other mammalian species (nonhuman primate, pig, and rabbit). This new tool requires none or minimal laboratory equipment, and the results are obtained within 15 min. When compared to the method of microagglutination, which was shown to be more specific than the enzyme-linked immunosorbent assay, the ICT had a sensitivity of 98.3% (58 positive sera were tested) and a specificity of 96.5% (58 negative sera were tested) on human sera. On animal sera, the overall sensitivity was 100% (22 positive sera were tested) and specificity was also 100% (70 negative sera were tested). This rapid test preferentially detects IgG antibodies that may occur early in the course of human tularemia, but further evaluation with human sera is important to prove that the ICT can be a valuable field test to support a presumptive diagnosis of tularemia. The ICT can also be a useful tool to monitor successful vaccination with subunit vaccines or live vaccine strains containing lipopolysaccharide (e.g., LVS) and to detect seropositive individuals or animals in outbreak situations or in the context of epidemiologic surveillance programs in areas of endemicity as recently recommended by the World Health Organization.