Regulatory T Cell Activity and Signs of T Cell Unresponsiveness in Bovine Paratuberculosis

Abstract

Johne’s disease, caused by infection with Mycobacterium avium subspecies paratuberculosis (MAP), is a wasting disease of ruminants displaying a long subclinical stage of infection followed by clinical disease characterized by severe diarrhea, wasting, and premature death. Immunologically, subclinical disease is characterized by a Th1 response effective at controlling intracellular infections such as that caused by MAP. In late subclinical disease, the Th1 response subsides and a non-protective Th2 response becomes prominent. One hypothesis for this shift in immune paradigm is that a population of MAP-reactive regulatory T cells (Tregs) develops during subclinical infection, limiting Th1-type responses to MAP antigens. To investigate this, we sought to accomplish the following: (1) determine if CD4+CD25− T cells exposed to MAP-infected macrophages develop a Treg phenotype, (2) develop a method to expand the relative abundance of Tregs in bovine peripheral blood lymphocyte populations, and (3) identify functional activities of expanded Tregs when combined with autologous peripheral blood mononuclear cells (PBMCs) and live MAP. We found that CD4+CD25− T cells exposed to MAP-infected macrophages from cows with Johne’s disease do not show signs of a Treg phenotype and appear unresponsive to MAP antigens. A method for Treg expansion was successfully developed; however, based on results obtained in the subsequent functional studies it appears that these Tregs are not MAP-specific. Overall, it seems that T cell unresponsiveness, rather than Treg activity, is driving the Th1-to-Th2 immune shift observed during Johne’s disease. Further, we have successfully developed a method to enrich non-specific bovine Tregs that exert suppressive effects against Th1 cytokine production.

Transcriptome Profiling of Bovine Macrophages Infected by Mycobacterium avium spp. paratuberculosis Depicts Foam Cell and Innate Immune Tolerance Phenotypes

“…This model has been challenged from time to time by subsequent studies, since the conversion to a Th2 humoral immune response is not the most accurate description of the progression of the disease (10-12). As JD progresses, T cell hyporesponsiveness is observed (11), which is also documented as T cell exhaustion (13,14) and/or anergy (12). Alternatively, it was suggested that another subtype of T cells, the regulatory T cells (Treg), play a regulatory role in JD (13-15).…”

Mycobacterium avium Subspecies paratuberculosis Drives an Innate Th17-Like T Cell Response Regardless of the Presence of Antigen-Presenting Cells

“…This is also true in MAP-infected monocytederived macrophages (MDMs) (primarily IL-1β, IL6, and IL-23) (33). Naïve helper T cells (CD4+ CD25-) from healthy JD-cows also show increased IL-17A mRNA expression when cocultured with autologous MAP-infected MDMs (32). In vivo, IL-17A and IL-6 are upregulated in early-stage MAP-infected lesions (Grade 1) (34), but not in late infection/clinical Johne's disease lesions, suggesting that Th17 responses to MAP are subject to the same T cell exhaustion that occurs with Th1like cells in late clinical JD (35).…”

“…Stimulation of peripheral blood mononuclear cells (PBMCs) from healthy JDcows with MAP increased the mean relative percent of T cell subtypes that expressed IL-23R (31). Furthermore, mRNAs encoding cytokines that direct T cells toward a Th17-like phenotype were upregulated in subclinical JD+ and even JD-PBMCs exposed to MAP in culture (IL-6, IL-1, IL-23, and IL-17A) (31,32). This is also true in MAP-infected monocytederived macrophages (MDMs) (primarily IL-1β, IL6, and IL-23) (33).…”

The immunoregulatory effects of co-infection with Fasciola hepatica : From bovine tuberculosis to Johne's disease

“…Th2 cells, in contrast, secrete IL-4, IL-5, IL-10 and IL-13, which induce M2 cells and activate B cells to produce immunoglobulins (Mills et al, 2000). While in cattle Johne's disease appears to follow this Th1/Th2 switching pattern, another profile is possible in sheep, where both IFN- and antibody production increase at the same time (Begg et al, 2011;De Silva et al, 2011;Roussey et al, 2014). It is worth mentioning that antibody production has also been detetced in early infection in cattle (Stabel et al, 2011;Waters et al, 2003Waters et al, , 1999.…”

“…However, in these studies both memory T and B cell proliferation and production of non-protective IgG1 antibody increased gradually throughout the subclinical stage of disease. Increased antibody levels are probably related to disease progression rather than switch to clinical disease given that animals with clinical Johne's disease show T and B cell unresponsiveness (Roussey et al, 2014;Waters et al, 1999). The role of antibody in BTB and Johne's disease is still unclear (Jacobs et al, 2016;Schiller et al, 2010).…”