Prostaglandin regulation of immune responses against coronavirus infections

Prostaglandins (PG) are ubiquitous lipid mediators that play key roles in pathophysiological responses to infections. They are considered to have both pro and anti-inflammatory roles depending upon the time of inflammation, the receptors that they bind to and the tissues that they act upon. Hence given their pleiotropic effects, a perfect balance between the pro and anti-inflammatory functions of PGs are required to ensure that a controlled timely immune response is elicited to mediate protection and to avoid immunopathology.

PGD₂ is one such PG that was reported to increase with age in the lungs of mice and to mediate an anti-inflammatory effect thereby blunting the immune response following Severe Acute Respiratory Syndrome – Coronavirus (SARS-CoV). Increase in PGD₂ with age incapacitates respiratory dendritic cells (rDC) to migrate from lungs to the draining lymph node following SARS-CoV infection due to down regulation of CCR7 (a receptor for chemokines CCL19/21). Migration of rDCs to draining lymph nodes requires high expression of CCR7 and it's binding to CCL19/21, a chemokine that mediates migration of dendritic cells along its gradient. Although increase in levels of PGD₂ might prove beneficial in high inflammatory conditions, it should be noted that high levels of such a potent anti-inflammatory mediator during the initiation of an immune response could prove detrimental. In chapter II of this thesis I show that age-related increases in oxidative stress result in the upregulation of a single phospholipase (PLA₂) group II D (G2D) (PLA₂G2D) with anti-inflammatory roles. PLA₂G2D functions by releasing Arachidonic acid (AA) from the lipid membrane, which will be further metabolized to other pro-resolving/ anti-inflammatory lipid mediators including PGD₂. I show that inducing oxidative stress in young mice as well as in human peripheral blood macrophages, results in the upregulation of PLA₂G2D (probably as a counter mechanism against oxidative stress). Also increase in the expression levels of this gene during the course of SARS-CoV infection results in the upregulation of PGD₂, which is completely abrogated in Pla2g2d^-/- mice. I also show Pla2g2d/- middle-aged mice have low levels of PGD₂ and that they are capable of mounting a strong immune response and survive the otherwise lethal SARS-CoV infection.

PGD₂ is also a major PG in the brain and its role has been investigated in many non-infectious setting such as stroke and Alzheimer' disease. The PGD₂ binding to one of its receptors DP1 has been shown to have primarily a neuro-protective role. In chapter III, I show that PGD₂/DP1 signaling has beneficial effects in the brain of mice infected with a neurotropic strain of murine hepatitis virus (MHV) (rj2.2). In agreement with the neuro-protective role of PGD₂, at least 60% of DP1^-/- mice succumb to a sublethal dose of rj2.2. rj2.2 infection in these mice is characterized by a delay in the induction of IFN I response and lower activation status of microglia and macrophages in the brain. I also show that abrogation of DP1 signaling results in global defects in the immune system response to infection. Notably, a genome wide expression analysis using microarray, shows that a gene, Pydc3 with putative inflammasome inhibiting function is upregulated in WT mice compared to DP1^-/- mice in the CD11b population of cells which primarily comprises microglia and macrophages. In line with the predicted function of Pydc3, DP1^-/- mice have higher frequency and number of IL-1β⁺ producing microglia in the brain. Studies are underway to determine the exact role of DP1 signaling in Pydc3 expression as well as the role of this gene in inflammasome function.

Overall these studies emphasize the immuno-modulatory roles of PGs in the context of a viral infection. Thus, altering the levels of these lipid mediators at appropriate times during the course of infection might prove useful as an effective therapeutic strategy to decide the fate of an infection.