Antibodies Induced by Lipoarabinomannan in Bovines: Characterization and Effects on the Interaction between Mycobacterium Avium Subsp. Paratuberculosis and Macrophages In Vitro

Modeling Paratuberculosis in Laboratory Animals, Cells, or Tissues: A Focus on Their Applications for Pathogenesis, Diagnosis, Vaccines, and Therapy Studies

Paratuberculosis is a chronic granulomatous enteritis caused by Mycobacterium avium subsp. Paratuberculosis that affects a wide variety of domestic and wild animals. It is considered one of the diseases with the highest economic impact on the ruminant industry. Despite many efforts and intensive research, paratuberculosis control still remains controversial, and the existing diagnostic and immunoprophylactic tools have great limitations. Thus, models play a crucial role in understanding the pathogenesis of infection and disease, and in testing novel vaccine candidates. Ruminant animal models can be restricted by several reasons, related to space requirements, the cost of the animals, and the maintenance of the facilities. Therefore, we review the potential and limitations of the different experimental approaches currently used in paratuberculosis research, focusing on laboratory animals and cell-based models. The aim of this review is to offer a vision of the models that have been used, and what has been achieved or discovered with each one, so that the reader can choose the best model to answer their scientific questions and prove their hypotheses. Also, we bring forward new approaches that we consider worth exploring in the near future.

Sheep vaccinated against paratuberculosis have increased levels of B cells infiltrating the intestinal tissue

Systemic immunisation delivered subcutaneously is currently used to control paratuberculosis, a chronic enteritis of ruminants caused by Mycobacterium avium subspecies paratuberculosis (MAP). These vaccines do not provide complete protection and a small cohort of animals still succumb to clinical disease. The aim of this study was to assess mycobacterial infection site-specific variations in immune cells in vaccinated sheep that did or did not develop the disease following controlled exposure to MAP. Immunohistochemical staining of terminal ileum demonstrated that vaccination increased infiltration of CD4 + T cells and B cells. Infiltration of large numbers of CD4 + T and B cells was also seen in sheep that successfully cleared infection. Vaccination promoted the polarisation of macrophages to an M1 activation state. The presence of certain cells at the site of infection, especially CD4 + T cells, is likely to contribute to vaccine success by increasing the speed and potency of the local immune response. Systemic immunisation against MAP can alter the composition of innate and adaptive immune cell populations at the predilection site for MAP infection in the ileum one year after vaccination. This informs understanding of the impact of vaccination at the site of infection and also the duration of vaccine-elicited changes. This information may assist vaccine development and allow targeting of protective immune responses in the gut of ruminants.

Antibodies from healthy or paratuberculosis infected cows have different effects on Mycobacterium avium subspecies paratuberculosis invasion in a calf ileal loop model

In this work, we used a calf ileal loop model to evaluate whether the preincubation of Mycobacterium avium subspecies paratuberculosis (MAP) with antibodies from healthy, MAP-positive or Lipoarabinomannan (LAM) immunized cows could affect the results of infection after 3.5 h. Bacterial load in tissue was assessed by Ziehl-Neelsen and by culture for each loop. MAP was detectable in all infected loops after 3.5 h.p.i.; although the presence of antibodies from MAP-positive cows significantly reduced bacterial load in loops as compared with antibodies from healthy donors (by Ziehl-Neelsen and culture, p-value < 0.003 and 0.0203, respectively). A possible direct effect of antibodies on MAP viability was shown to be not significant. Severity of histopathologic changes induced by MAP infection also varied according to the pretreatment: MAP induced less changes when inoculated in the presence of antibodies from MAP-positive cows as compared with antibodies from healthy donors. Overall, our results show that the presence of antibodies from MAP-positive cows reduced MAP invasion and consequent early histological changes in this ileal short-term loop model. These results may suggest a protective role of antibodies in the response against MAP at the portal of entry in cattle.

The humoral immune response is essential for successful vaccine protection against paratuberculosis in sheep

Background

The role played by the humoral immune response in animals vaccinated against a mycobacterial disease such as paratuberculosis, is not well understood. Sheep vaccinated against Mycobacterium avium subsp. paratuberculosis (MAP) can still become infected and in some cases succumb to clinical disease. The strength and location of the humoral immune response following vaccination could contribute to the ability of sheep to clear MAP infection. We examined the peripheral antibody response along with the localised humoral response at the site of paratuberculosis infection, the ileum, to better understand how this contributes to MAP infection of sheep following vaccination and exposure.

Results

Through assessing MAP specific serum IgG1 and IgG levels we show that the timing and strength of the humoral immune response directly relates to prevention of infection following vaccination. Vaccinated sheep that subsequently became infected had significantly reduced levels of MAP specific serum IgG1 early after vaccination. In contrast, vaccinated sheep that did not subsequently become infected had significantly elevated MAP specific serum IgG1 following vaccination. Furthermore, at 12 months post MAP exposure, vaccinated and subsequently uninfected sheep had downregulated expression of genes related to the humoral response in contrast to vaccinated infected sheep where expression levels were upregulated.

Conclusions

The timing and strength of the humoral immune response following vaccination against paratuberculosis in sheep directly relates to subsequent infection status. An initial strong IgG1 response following vaccination was crucial to prevent infection. Additionally, vaccinated uninfected sheep were able to modulate that response following apparent MAP clearance, unlike vaccinated infected animals where there was apparent dysregulation of the humoral response, which is associated with progression to clinical disease.

Early IgG2 in calves experimentally infected with Mycobacterium avium subsp. paratuberculosis

The diagnosis of the early stages of paratuberculosis, caused by Mycobacterium avium subsp. paratuberculosis (Map), is a cumbersome task. In this study, an experimental Map-infection model of calves was used to improve the knowledge of early antibody response and to evaluate different in-house ELISAs in the detection of subclinical paratuberculosis. Calves were challenged with Map strain _IS900-RFLPA (n = 3) or Map strain _IS900-RFLPC (n = 2) (Argentinean isolated strains) or mock infected (n = 3), and their specific humoral response was evaluated. The diagnostic ELISA (IgG against Map protoplasmic antigen; PPA) could not detect the infection throughout the experimental period (180 days post-infection; dpi), whereas the IgG2/PPA-ELISA was able to identify infected calves at least once during the experiment. In addition, the use of crude Map extract detected most of the infections from 60 dpi onwards. Antibodies were also characterized by immunoblot: IgG2-reactivity to antigens of molecular weight lower than 50 kDa was detected in all infected calves. The experimental Map-infection model of calves used allows the study of the early humoral immune response in paratuberculosis. The evaluation of IgG2 specific to antigens lighter than 50 kDa emerges as an interesting alternative in calves naturally infected with paratuberculosis.

Evidence of a pro-apoptotic effect of specific antibodies in a bovine macrophage model of infection with Mycobacterium avium subsp. paratuberculosis

Mycobacterium avium subspecies paratuberculosis (MAP) is the causative agent of Johne's disease (JD), a chronic granulomatous enteritis in ruminants. Understanding the protective immune response following infection is crucial to improve the diagnosis and the development of vaccines against this disease. The goal of this work was to assess whether specific antibodies were able to modulate the macrophage response to MAP infection by evaluating apoptosis and TNF-α secretion in an in vitro model. Sera from healthy (n=2), MAP-infected (n=3) and lipoarabinomannan (LAM)-immunized (n=3) bovines were evaluated. LAM was chosen as immunogen due to its relevant role in mycobacterial pathogenesis. We demonstrated by two different techniques (Acridine Orange/Ethidium Bromide microscopy and Annexin V/7-Amino-Actinomycin D flow cytometry) that the immune sera from both, MAP-infected and LAM-immunized bovines, significantly increased macrophage apoptosis in infected cultures. Comparable levels of apoptosis were detected when MAP was pre-incubated with purified specific antibodies instead of whole serum. Furthermore, this effect was accompanied by a significantly higher secretion of TNF-α. These results strongly suggest that specific antibodies could limit the impact of MAP on the apoptosis of bovine cells. This work would contribute to elucidate the role of the specific antibody response in bovine JD and its prevention.

Fasciola hepatica - the pilot study of in vitro assessing immune response against native and recombinant antigens of the fluke

Fasciola hepatica is a liver fluke that infects 2.4 million of people and causes great economical loss in animal production. To date a 100% effective vaccine has not been developed and the disease is controlled by drug therapy. Great efforts are put into development of effective vaccine against parasite what is difficult since Fasciola spp. (like other helmints) during evolutionary process has developed sophisticated and efficient methods to evade immune response. During preliminary experiments it is convenient to use cell lines which are relatively cheap and allow for reproducible comparison of results between laboratories. We stimulated BOMA (bovine monocyte/macrophage cell line) and BOMAC (bovine macrophage cell line) with native or recombinant antigens of Fasciola hepatica and assessed IFN-γ, IL-4 and TNF-α level upon stimulation. We observed diminished secretion of proinflammatory TNF-α in LPS activated BOMA cells stimulated with Excretory/Secretory products of adult fluke (Fh-ES). We also observed greater changes in gene expression in LPS activated BOMA cells than in non activated BOMA cells upon stimulation using Fh-ES. The results show possibility of using cell lines for in vitro research of bovine immune response against liver fluke, although this model still requires validation and further characterization.

Detection of Bovine IgG Isotypes in a PPA-ELISA for Johne's Disease Diagnosis in Infected Herds

Johne's Disease or Paratuberculosis is a chronic granulomatous enteritis disease affecting ruminants. Detection of subclinically infected animals is difficult, hampering the control of this disease. The aim of this work was to evaluate the performance of detection of IgG isotypes in a PPA-ELISA to improve the recognition of cattle naturally infected with Map in different stages. A total of 108 animals from Tuberculosis-free herds were grouped as follows: exposed (n = 30), subclinically infected (n = 26), clinically infected (n = 14), and healthy controls (n = 38). Receiver-operating characteristic (ROC) curves of isotypes/PPA-ELISAs were constructed and areas under the curves were compared to evaluate the performance of each test. Our study demonstrated that the conventional PPA-ELISA (detecting IgG) is the best to identify clinically infected animals with high sensitivity (92.9%) and specificity (100%). Meanwhile, IgG2/PPA-ELISA improved the number of subclinically infected cattle detected as compared with conventional IgG/PPA-ELISA (53.8 versus 23.1%). In addition, it had the maximum sensitivity (65.0%, taking into account all Map-infected cattle). In conclusion, the combination of IgG and IgG2/PPA-ELISAs may improve the identification of Map-infected cattle in different stages of disease. The usefulness of IgG2 detection in serological tests for Johne's Disease diagnosis should be further evaluated.