Background: The Gram-positive bacterium, Staphylococcus aureus remains a leading cause of both community and hospital-acquired pneumonia in the United States. Increased prevalence of antimicrobial resistance, as seen in methicillin-resistant Staphylococcus aureus (MRSA), accentuates the need to design improved therapies for bacterial pneumonia. Effective innate immune activation and response in the lungs are critical for bacterial clearance. Neutrophils are the primary defenders of innate immunity. Increasing numbers of neutrophils are produced in the bone marrow and rapidly released into the blood in order to migrate to inflamed tissues during bacterial pneumonia. Paradoxically, excessive neutrophil influx can lead to extensive tissue damage. Therefore, modulation of key molecules that regulate neutrophil recruitment is crucial to designing improved therapies that augment innate immunity while attenuating excessive pulmonary inflammation during bacterial pneumonia. 

CD38 is a multifunctional protein endowed with both receptor and enzymatic functions. Its expression is constitutive or inducible in immunocytes and other cell types during inflammation. So far, it is well known that CD38 regulates numerous infection-induced inflammatory processes, ranging from cell activation, cell differentiation, and cell apoptosis to the induction of adaptative immune responses. Nevertheless, the role of CD38 in neutrophil recruitment against S. aureus-induced pneumonia is largely unknown. Based on literature review, we propose the hypothesis that CD38 is a central regulator of neutrophil recruitment during S. aureus-induced pneumonia through modulation of signaling cascade events involving Toll-like receptors (TLRs) stimulation, transcription factor activation, cytokine/chemokine secretion as well as adhesion molecule regulation.

Methods and Results: We used a pre-clinical mouse model to investigate the mechanistic role of CD38 in neutrophil recruitment during pneumonia caused by S. aureus (USA300). CD38^-/- mice and their WT controls were infected intratracheally with a sub-lethal dose (5x10⁷ CFU/mouse) of S. aureus. We performed bronchoalveolar lavage fluid (BALF) cell phenotyping and analyzed bacterial burden in the lungs and in extrapulmonary organs after infection. We also quantified the level of cytokines in BALF. Compared to WT counterparts, CD38^-/- mice displayed enhanced inflammation, which was associated with enhanced neutrophil influx in BALF and higher bacterial burden in the lungs following infection. In addition, CD38^-/- mice showed increased levels of cytokines, including TNF-α, IL-6, and IL-17a, in BALF at 24-hours post-infection. Moving forward, we will use flow cytometry and western blotting to determine whether CD38 regulates TLR2 and 9 expression and NF-κB activation after S. aureus infection. Furthermore, we will assess whether CD38 regulates the expression of adhesion molecules, including VCAM-1 and ICAM-1, following infection by western blotting.

Conclusions: This knowledge will enhance our understanding of how CD38 regulates neutrophil recruitment during S. aureus-induced pneumonia and potentially facilitate the development of therapeutic treatment strategies to augment neutrophil-mediated host defense against Gram-positive bacterial pneumonia.