Prostaglandin D₂ as a mediator of lymphopenia and a therapeutic target in COVID-19 disease

Abstract

A characteristic feature of COVID-19 disease is lymphopenia. Lymphopenia occurs early in the clinical course and is a predictor of disease severity and outcomes. The mechanism of lymphopenia in COVID-19 is uncertain. It has been variously attributed to the release of inflammatory cytokines including IL-6 and TNF-α; direct infection of the lymphocytes by the virus; and rapid sequestration of lymphocytes in the tissues. Additionally, we postulate that prostaglandin D₂ (PGD₂) is a key meditator of lymphopenia in COVID-19. First, SARS-CoV infection is known to stimulate the production of PGD₂ in the airways, which inhibits the host dendritic cell response via the DP₁ receptor signaling. Second, PGD₂ is known to upregulate monocytic myeloid-derived suppressor cells (MDSC) via the DP₂ receptor signaling in group 2 innate lymphoid cells (ILC2). We propose targeting PGD₂/DP₂ signaling using a receptor antagonist such as ramatroban as an immunotherapy for immune dysfunction and lymphopenia in COVID-19 disease.

Lymphopenia is one of the characteristic features of COVID-19 disease in adults, and a predictor of morbidity and mortality [1], [2]. Patients with lymphopenia have more severe disease; correction of lymphopenia correlates with recovery from severe disease, while severe and sustained lymphopenia is associated with fatal outcomes [1], [2]. Consistent with higher mortality in adults with COVID-19, lymphopenia is more common in adults than children. In meta-analyses, 15% of the 1667 children, and over 50% of the 3,062 adults had lymphopenia [3], [4]. Lymphopenia was also observed in 46% of the 80 children, and about 70% of 138 adults in SARS-CoV 2003 infection, and lymphopenia was reported to persist for as long as 1 to 2 years [5], [6], [7].

The mechanisms underlying lymphopenia during SARS-CoV and SARS-CoV-2 infections remain unclear. Lymphocytes have minimal expression of angiotensin converting enzyme 2 (ACE2) [8], [9]. SARS-CoV and SARS-CoV-2 have not been demonstrated to directly infect lymphocytes [9]. Peripheral T lymphocytes, both CD4+ and CD8+, are rapidly reduced in acute SARS-CoV infection possibly due to lymphocytic infiltration and sequestration in specific target organs [10]. Lymphopenia, in the later stages of COVID-19 illness, may have been mediated by thymic involution and atrophy induced by hyperinflammation and cytokine release comprising of IL-6, TNF-α, and IL-1 [11]. However, lymphopenia has been reported to occur concurrently with the onset of clinical symptoms in COVID-19 [1]. We postulate that lymphopenia observed at the onset or during the early stages of COVID-19 illness is caused by increased generation of prostaglandin D₂ by the respiratory epithelium.

Prostaglandin D₂ (PGD₂) is a key eicosanoid generated in respiratory infections. Severe bronchiolitis in infants caused by respiratory syncytial virus (RSV) leads to marked increase in PGD₂ in the airways [12]. Mice infected with SARS-CoV also exhibit significant increases in PGD₂ concentrations in the bronchoalveolar lavage fluid [13]. SARS-CoV respiratory infection stimulates PGD₂ production by increased expression of phospholipase A₂ group IID (PLA₂G2D), cyclooxygenase-2 (COX-2), and hematopoietic PGD₂ synthase (hPGDS) [14]. Furthermore, protein sequences in the spike and nucleocapsid proteins of SARS-CoV activate the expression of the COX-2 gene [15], [16]. Increased expression of PLA₂G2D and hPGDS genes also occurs with aging, leading to increased levels of PGD₂ in the airways of the elderly [13]. Compared to the 6-week old mice, there is a 300–400% increase in the airways’ PGD₂ levels in 12-month old and 22-month old mice [13]. PGD₂ action is mediated by binding to two G-protein coupled receptors, D-prostanoid receptor 1 (DP₁); and D-prostanoid receptor 2 (DP₂), formerly known as chemoattractant receptor-homologous molecule on T helper type 2 cells (CRTH2) [17]. PGD₂ has been reported to affect the host’s innate and adaptive immune responses to viruses including SARS-CoV as described below.

Early in infection, activated respiratory dendritic cells (rDC) undergo a maturation process that includes upregulation of costimulatory ligands, antigen-presenting complexes, and importantly, chemokine receptors such as C–C chemokine receptor type 7 (CCR7) [13]. The elevated levels of chemokine receptors facilitate migration of antigen-bearing rDCs to the local draining lymph nodes (DLNs) in the mediastinum where they participate in initiating adaptive host immune response to the respiratory virus. PGD₂/DP₁ signaling in the airway epithelial cells leads to the inhibition of CCR7 which suppresses rDC migration to draining lymph nodes. This leads to impairment of T lymphocyte priming and maturation, thereby leading to lymphopenia [13], [18]. Second, PGD₂/DP₂ signaling stimulates Group 2 innate lymphoid cells (ILC2) and T helper 2 (Th2) cells to secrete interleukin-13 (IL-13). IL-13 upregulates monocyte-macrophage derived suppressor cells (MDSC), which downregulates the T-lymphocyte response, causing lymphopenia [19], [20], [21]. MDSCs mediated impairment of pathogen specific adaptive immune responses has been demonstrated with Hemophilus influenzae respiratory infection [22]. Interestingly, ILC2, despite their scarcity, are the dominant innate lymphoid cell population in the lung, indicating a key role as first responders and amplifiers upon immune challenge at this site [23].

Based on the above findings, we hypothesize that an increase in airway PGD₂ levels initiates lymphopenia in COVID-19 (Fig. 1 ). We propose that antagonism of PGD₂ synthesis or signaling can prevent lymphopenia or promote recovery of lymphocyte counts in COVID-19 disease. However, suppression of PGD₂ synthesis will inhibit PGD₂/DP₁ signaling which has been demonstrated to attenuate inflammation and reduce vascular permeability [24], [25]. Therefore, selective targeting of PGD₂/DP₂ signaling, while sparing PGD₂/DP₁ axis, is necessary to restore immune dysfunction during the symptomatic phase of COVID-19. Ramatroban is a potent, reversible, and selective antagonist of PGD₂/DP₂ receptors that has been shown to inhibit PGD₂ stimulated IL-13 secretion, with an IC-50 of 118 nM [17], [20]. Ramatroban has been used orally as a treatment for allergic rhinitis in Japan for the past 20 years.[26] Given the global disease burden of the COVID-19 pandemic, there is an urgent need to examine the role of eicosanoids including PGD₂ in the pathogenesis of the disease, and to investigate the potential immunotherapeutic role of PGD₂ antagonists such as ramatroban.

Fig. 1.

Proposed mechanism of lymphopenia in patients with COVID-19. A host specific, exuberant PGD₂ response early in infection, initiates DP₁ signaling, which inhibits the dendritic cell function by downregulating CCR7, leading to a weak T cell response. PGD₂/DP₂ signaling stimulates respiratory ILC2 and Th2 cells, which secrete IL-13. IL-13 stimulates proliferation of MDSC cells, thereby downregulating the pathogen specific T cell responses. Excessive PGD₂ action via DP₁ receptors during the incubation period and DP₂ receptors during the symptomatic stage leads to lymphopenia. Lymphopenia is a predictor of morbidity and mortality in COVID-19.

Disclosure of Potential Sources of Conflict of Interest

AG has filed three provisional patent applications for use of PGD₂ and thromboxane A₂ antagonists, including ramatroban, as a treatment for COVID-19 (Application numbers: 63/003,286 filed on March 31; 2020; 63/005,205 filed on April 3, 2020; and 63/027,751 filed on May 2, 2020). Other authors have not declared conflict of interest. Ramatroban (Baynas®) was approved in Japan for allergic rhinitis in 2000.

Appendix A. Supplementary data

The following are the Supplementary data to this article: