Persistent infection due to a small-colony variant of Burkholderia pseudomallei leads to PD-1 upregulation on circulating immune cells and mononuclear infiltration in viscera of experimental BALB/c mice

Abstract

BackgroundMelioidosis is a neglected tropical disease endemic across South East Asia and Northern Australia. The etiological agent, Burkholderia pseudomallei (B.pseudomallei), is a Gram-negative, rod-shaped, motile bacterium residing in the soil and muddy water across endemic regions of the tropical world. The bacterium is known to cause persistent infections by remaining latent within host cells for prolonged duration. Reactivation of the recrudescent disease often occurs in elders whose immunity wanes. Moreover, recurrence rates in melioidosis patients can be up to ~13% despite appropriate antibiotic therapy, suggestive of bacterial persistence and inefficacy of antibiotic regimens. The mechanisms behind bacterial persistence in the host remain unclear, and hence understanding host immunity during persistent B. pseudomallei infections may help designing potential immunotherapy.Methodology/Principal findingsA persistent infection was generated using a small-colony variant (SCV) and a wild-type (WT) B. pseudomallei in BALB/c mice via intranasal administration. Infected mice that survived for >60 days were sacrificed. Lungs, livers, spleens, and peripheral blood mononuclear cells were harvested for experimental investigations. Histopathological changes of organs were observed in the infected mice, suggestive of successful establishment of persistent infections. Moreover, natural killer (NK) cell frequency was increased in SCV- and WT-infected mice. We observed programmed death-1 (PD-1) upregulation on B cells of SCV- and WT-infected mice. Interestingly, PD-1 upregulation was only observed on NK cells and monocytes of SCV-infected mice. In contrast, cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) downregulation was seen on NK cells of WT-infected mice, and on monocytes of SCV- and WT-infected mice.Conclusions/SignificanceThe SCV and the WT of B. pseudomallei distinctly upregulated PD-1 expression on B cells, NK cells, and monocytes to dampen host immunity, which likely facilitates bacterial persistence. PD-1/PD-L1 pathway appears to play an important role in the persistence of B. pseudomallei in the host.

Experimental exposure of Burkholderia pseudomallei crude culture filtrate upregulates PD-1 on T lymphocytes

“…In an acute infection, such an inhibitory signal would limit the number of effector T cells during T cell expansion [ 30 ], whereas during chronic infection, due to persistent exposure to antigens, this interaction renders the T cells unresponsive, leading to T cell exhaustion [ 31 ]. Increasing evidence suggests that PD-1 has a role in the inhibition of effector T cell responses in persistent Mycobacterium tuberculosis [ 32 ] and persistent viral infections [ 33–35 ], which we demonstrated in a murine model of persistent B. pseudomallei infection [ 36 ]. Upregulation of PD-L1 on polymorphonuclear neutrophils (PMNs) of melioidosis patients has been shown to impair T cell functions [ 37 ].…”

“…PD-1 was recently shown to be upregulated in persistent B. pseudomallei infections in mice [ 36 ], which nonetheless has not been proven in humans. Here, healthy PBMCs were exposed to B. pseudomallei and B. thailandensis antigens in vitro and, following 18 h of incubation, cells were assessed for PD-1 and TIM-3 expression.…”

“…Recent research shows that the PD-1 pathway is emerging as an important mechanism exploited by many viruses and intracellular bacteria to dampen T cell responses [ 33–35 ]. We previously showed that PD-1 is highly upregulated during persistent B. pseudomallei infection in a murine model [ 36 ]. Although animal studies allow a closer approximation to human responses, we sought to validate if such PD-1 upregulation is translated in humans.…”

Release of pro-inflammatory cytokines TNF-α, IFN-γ and IL-6 by Burkholderia pseudomallei- stimulated peripheral blood mononucleocytes of acute myeloid leukemia patients

“…Three different strains of B. pseudomallei used were OB, OS and K9. OS is a small colony variant (SCV) which is associated with persistent infection and antibiotic resistance whereas OB is its wild-type (See et al, 2017). K9 was isolated from a melioidosis patient and had been widely used in experimental B. pseudomallei.…”

Hijacking of the Host’s Immune Surveillance Radars by Burkholderia pseudomallei

“…In addition, using a mice model, See et al. (2017) ( 115 ) characterized the regulation of PD-1 and CTLA-4 levels on B cells, NK cells, and monocytes and confirmed that persistent infection with B. pseudomallei small colony variants (SVCs) can concomitantly cause PD-1 upregulation on B cells, NK cells and monocytes signifying host immune exhaustion. The SCVs were found to up-regulate the PD-1 on NK cells and monocytes in contrast to its WT counterpart.…”