Sepsis is a clinical syndrome that complicates severe infections. It is characterized by the cardinal signs of inflammation (e.g. vasodilation, leukocytosis, increased microvascular permeability) occurring in tissues that are remote from the site of an infection. Current theories about the onset and progression of the sepsis syndrome focus on dysregulation of inflammatory responses, including the possibility that a massive and uncontrolled release of pro-inflammatory mediators initiates a chain of events that lead to widespread tissue injury. The degree of immune dysfunction is thought to correlate with the severity of the sepsis syndrome. Sepsis syndrome can range from sepsis, to severe sepsis, septic shock, and multiple organ dysfunction syndrome (MODS). The mortality associated with each of these is estimated to be 16%, 20%, 46%, \>80%, respectively. The annual incidence of sepsis syndrome exceeds 1.6 million cases in the United States alone.

Recently, cells of the innate and adaptive immune system have been shown to express the vitamin D receptor. Vitamin D appears to be necessary for interferon-γ dependent T cell responses to infection. In low vitamin D states, dysfunctional macrophage activity becomes evident. Vitamin D is also an important link between Toll Like Receptor (TLR) activation and antibacterial response. Human macrophages stimulated by TLR induce: 1) vitamin D receptor expression; 2) conversion of 25(OH)D to its most biologically active form of 1,25-dihydroxyvitamin D; and 3) production of cathelicidin (LL-37), an endogenous antimicrobial peptide with potent activity against bacteria, viruses, fungi, and mycobacteria. LL-37 is highly expressed in both the plasma and at natural barrier sites (e.g. skin, gut, lungs) and may represent an important first-line of defense for the innate immune system.

In humans, cholecalciferol (vitamin D3) is either obtained through the diet or synthesized by skin upon exposure to ultraviolet B (UVB) radiation. Cholecalciferol is converted to 25(OH)D in the liver or by cells of the immune system. Serum 25(OH)D can be measured with relative ease and is the most abundant vitamin D metabolite. It is therefore, often used as a proxy for total body vitamin D status and 25(OH)D levels \<30 ng/mL characterize an insufficient state. A growing body of evidence suggests that a significant proportion (50-90%) of critically ill patients may have insufficient 25(OH)D levels during admission to the intensive care unit (ICU). 25(OH)D insufficiency, in turn, appears to be associated with a higher risk of mortality in critically ill patients. However, randomized, placebo-controlled trials (RCTs) aimed at studying the effect of vitamin D supplementation in critical illness are limited and have largely focused on superficial assessments of vitamin D status. While it is known that septic patients have nearly universally low 25(OH)D levels and that the vitamin D levels are inversely correlated with the severity of sepsis, little is known regarding the effects of vitamin supplementation in this patient cohort. Therefore, our goal is to determine whether vitamin D supplementation in patients highly suspected of sepsis syndrome may be effective in optimizing 25(OH)D levels and in improving host production of the antimicrobial polypeptide LL-37.