The focus of the Hotchkiss Lab is defining new methods for the treatment of sepsis, a highly lethal disorder which occurs during severe overwhelming infection. Sepsis is the host response that occurs due to the presence of bacteria and/or their products within the bloodstream. The host response to sepsis can be manifested in a number of ways, including increased or decreased body temperature, increased heart rate, elevated white blood cell count, rapid respiratory rate, altered mental status, shock, and multiple organ dysfunction. Patients with a severe life-threatening form of sepsis in which there is evidence of inadequate organ perfusion (for example, shock, decreased renal function, depressed mental state, etc.) are stated to be in septic shock.
Sepsis and the resultant multiple organ failure that it induces is the most common cause of death in most intensive care units in developed countries. There are over 220,000 deaths annually due to sepsis in the United States alone. The primary area of investigation of the laboratory has been directed at prevention of apoptotic cell death of immune effector cells. Our laboratory was the first to show that apoptosis is a major cause of death in patients dying of sepsis (Crit Care Med 27:1230 1999). Autopsy studies of patients dying of sepsis showed that sepsis induced massive apoptosis of immune effector cells including T and B lymphocytes and dendritic cells (Figure 1).
The findings from our autopsy study of adult patients have been confirmed in two other important studies in pediatric patients and neonatal patients dying of sepsis. The widespread death of cells of the innate and adaptive immune system is the central pathophysiologic event in sepsis because of the systemic effects on the functioning of the immune system (Figure 2).
Septic patients develop a state of “immunoparalysis” rendering them unable to eradicate the primary initiating infection and making them highly susceptible to secondary hospital acquired infections.
Our laboratory was the first to demonstrate that prevention of apoptosis in sepsis could improve survival (J. Immunology 162:4148 1999). Mice whose lymphocytes overexpressed the anti-apoptotic protein Bcl-2 had a greater that 3-fold improvement in survival compared to wild type mice with sepsis. Subsequently, numerous other laboratories have shown that prevention of apoptosis by a variety of independent methods results in improved survival in sepsis.
Currently, the laboratory is focused on identifying mechanisms of cell death in sepsis and developing therapies to prevent this critical pathologic event. The list of current research projects within the laboratory include the following:
Identify the death stimuli and molecular pathways involved in sepsis-induced apoptosis via use of transgenic and knockout mice.
Mice that overexpress anti-apoptotic molecules or have knockout of pro-apoptotic molecules are employed as described in a recent publication that investigated the contribution of the death receptor and mitochondrial mediated pathways (Faseb J. 21:708 2007). In addition to apoptosis, our lab is currently examining the role and inter-relationship of autophagy and apoptosis in sepsis.
Inhibition of sepsis-induced immune effector cell apoptosis.
The laboratory is currently testing several strategies to block sepsis-induced death of immune effector cells. A promising method is use of anti-apoptotic cytokines that act to prevent cell death by increasing anti-apoptotic Bcl-2 family members and decreasing pro-apoptotic Bcl-2 family members. Our group has shown that several cytokines have potent abilities to block cell death in lymphocytes, dendritic cells, and natural killer cells. We are currently testing these cytokines for their ability to improve survival in selected models of sepsis.
Reversal of the “immunoparalysis” in sepsis
Recent clinical studies have established that patients with sepsis are profoundly immunosuppressed resulting in development of secondary infections that are often the ultimate cause of death. Our group is examining methods to reengage the immune effector cells of the innate and adaptive immune system. Strategies include blockade of negative co-stimulator molecules and administration of immunostimulatory cytokines. We are currently testing a number of compounds in selected models of sepsis.
Development of “humanized mice” for testing promising therapies
The ability to study human disease is limited due to both ethical and technical concerns. The development of “humanized” mice over the last five years has provided researchers with the remarkable ability to study human cells and tissues in an in vivo setting. “Humanized” mice are made by engrafting human CD34+ hematopoietic stem cells into immuno-deficient mice. After engraftment, mice develop human myeloid and lymphoid lineages, providing a surrogate to study the effects of infection, autoimmunity, transplantation, etc. on the human immune system. Our lab has generated “humanized” mice and we are employing these mice for many of the sepsis studies.