Soluble Programmed Cell Death Ligand 1 (Spd-L1) among Febrile Patients with Exposure to Selected Pathogens

Abstract

Introduction: Soluble programmed cell death ligand -1(sPD-L1) is an immune inhibitory checkpoint protein shown to play a pivotal role in maintaining immune homeostasis. However, disrupting the interaction between sPD-L1 and its receptor can result in the evasion of immune surveillance by many cancers and poor prognosis in various T-cell-mediated infectious diseases. Therefore, understanding the role of sPD-L1 is critical especially in infectious diseases. However, knowledge in this area is lacking in most endemic infections, which may impede the development of new diagnostic methods, therapeutics, and vaccines for these infections. Suggesting the need to investigate the possible association and potential role of sPD L1 levels in common endemic infections. Also, most studies on PD-L1 involved the use of whole blood samples in investigating membrane-bound PD-L1, with the use of flow cytometry. This technique may be challenging in some Sub-Saharan African countries, as it is expensive, requires trained professionals, and is laborious. Therefore, having a study which uses a simpler technique such ELISA in comparing sPD-L1 levels in both blood and urine to see if urine samples alone will be sufficient for assessing sPD-L1 would be beneficial, as urine is an easy clinical sample to obtain, and collection is less invasive than blood. The present study investigated the expression of sPD-L1 in the blood and urine of febrile patients with proof of exposure to selected pathogens. Methodology: This was a longitudinal study involving baseline febrile patients with exposure to selected pathogens and post-treatment on cohorts three weeks (21 days) after treatment. Both whole blood and urine samples were collected from consented patients with febrile illness who presented to Ledzokuku-Krowor Municipal Assembly (LEKMA) hospital. A total of 5 ml of whole blood was collected into EDTA tubes, out of which 2 ml was used in the screening of patients for exposure to selected pathogens and full blood count analysis. Plasma was separated from the remaining 3 ml by centrifugation. The concentration of plasma and urine sPD-L1 was measured using a commercial ELISA following the manufacturer´s instructions. The optical density (OD) was measured spectrophotometrically and a four (4) parameter logistic (4-PL) standard curve was generated using an online platform, and R2-values above 0.9 were considered acceptable. Results: Exposure to Plasmodium species, Salmonella Typhi, HSV-1, HSV-2, Toxoplasma gondii, HIV, hepatitis B virus, and syphilis was 7.08%, 1.5%, 52.8%, 46.17 %, 11%, 1.1%, 1.1%, and 2.2%, respectively. From this study, the plasma and urine concentrations of sPD-L1were significantly higher in patients than the controls prior to treatment (all p < .001); also, significantly higher sPD-L1 was seen in patients before treatment compared to patients after treatment (all p < .001), suggesting that sPD-L1 levels are upregulated during infection. In addition, no statistical difference was observed in sPD-L1 concentrations between malaria patients and patients with malaria and typhoid immunoglobulins, nor between controls and patients with typhoid immunoglobulins only (All p >.05), suggesting that typhoid immunoglobins may not necessarily contribute to elevated levels of sPD-L1. Also, there is no statistical difference in sPD-L1 levels in plasma and urine of patients with malaria only and patients with malaria and HSV immunoglobulins (p = .818 and p = 426, respectively, all p < .05), suggesting that HSV antibodies may also not contribute to further upregulated expression of sPD-L1 in the presence of Plasmodium infection. A positive correlation was established between Plasmodium parasite density, plasma sPD-L1 (rho =.554, p =.001) and urine sPD-L1 (rho =.412, p =.005). In addition, the present study found a correlation between sPD-L1 and hemoglobin levels, WBCs, platelets, lymphocytes, neutrophils, and eosinophils. Conclusion: This study demonstrated that plasma and urine sPD-L1 are detectable during systemic infections such as Plasmodium spp. infection, with patients having higher concentration at pre-treatment than post-treatment and controls. Our findings have also shown that upregulated expression of sPD-L1 in malaria patients is associated with increased Plasmodium parasite density. The present study has also shown for the first time that urine samples alone are enough to assess sPD-L1 expression in malaria patients.