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.

Activation of immunomodulatory pathways in response to invasive fungi can impair clearance and promote persistent infections. The Programmed Cell Death Protein-1 (PD-1) signaling pathway inhibits immune effector responses against tumors and immune checkpoint inhibitors that block this pathway are being increasingly used as cancer therapy. The objective of the current study was to investigate whether this pathway contributes to persistent fungal infection and to determine whether anti-PD-1 antibody treatment improves fungal clearance. Studies were performed using C57BL/6 mice infected with a moderately virulent strain of Cryptococcus neoformans (52D) which resulted in prolonged elevations in fungal burden and histopathologic evidence of chronic lung inflammation. Persistent infection was associated with increased and sustained expression of PD-1 on lung lymphocytes, including a mixed population of CD4+ T cells. In parallel, expression of the PD-1 ligands, PD-L1 and PD-L2, was similarly upregulated on specific subsets of resident and recruited lung dendritic cells and macrophages. Treatment of persistently-infected mice for four weeks by repetitive administration of neutralizing anti-PD-1 antibody significantly improved pulmonary fungal clearance. Treatment was well tolerated without evidence of morbidity. Immunophenotyping revealed that anti-PD-1 antibody treatment did not alter immune effector cell numbers or myeloid cell activation. Treatment did reduce gene expression of IL-5 and IL-10 by lung leukocytes and promoted sustained upregulation of OX40 by Th1 and Th17 cells. Collectively, this study demonstrates that PD-1 signaling promotes persistent cryptococcal lung infection and identifies this pathway as a potential target for novel immune-based treatments of chronic fungal disease.

This review addresses specific regulatory mechanisms involved in the host immune response to fungal organisms. We focus on key cells and regulatory pathways involved in these responses, including a brief overview of their broader function preceding a discussion of their specific relevance to fungal disease. Important cell types discussed include dendritic cells and regulatory T cells, with a focus on specific studies relating to their effects on immune responses to fungi. We highlight the interleukin-10, programmed cell death 1, and cytotoxic T lymphocyte-associated protein 4 signaling pathways and emphasize interrelationships between these pathways and the regulatory functions of dendritic cells and regulatory T cells. Throughout our discussion, we identify selected studies best illustrating the role of these cells and pathways in response to specific fungal pathogens to provide a contextual understanding of the tightly-controlled network of regulatory mechanisms critical to determining the outcome of exposure to fungal pathogens. Lastly, we discuss two unique phenomena relating to immunoregulation, protective tolerance and immune reactivation inflammatory syndrome. These two clinically-relevant conditions provide perspective as to the range of immunoregulatory mechanisms active in response to fungi.