ROLE OF CELLULAR EVENTS IN PATHOPHYSIOLOGY OF SEPSISUNDER THE SUPERVISION OF DR. PRANITA SARANGI

BY:

AGRAJ SHARMA

12111005

B TECH.

BIOTECHNOLOGY

WHAT IS SEPSIS

Sepsis is systemic inflammatory response to an infection. Sepsis develops due to a dysfunctional host immune response to an uncontrolled infection. In North America, approximately 750,000 cases of sepsis occur each year, with mortality ranging from 30% to 50%

STAGES IN SEPSIS

PRO-INFLAMMATORY STAGE The initial pro-inflammatory, or the cytokine storm stage,

is responsible for the recruitment of innate immune cells such as macrophages and neutrophils in response to an ongoing infection.

At this stage, recruited macrophages/monocytes secrete pro-inflammatory cytokines, chemokines, reactive oxygen species (ROS) via activation of toll-like receptors (TLRs) and NFκB pathway, which are very critical for the killing of bacteria.

This stage is primarily responsible for conveying appropriate signals to the cells of the adaptive immunity to intensify the immune responses and facilitate pathogen clearance.

IMMUNOSUPPRESIVE STAGE It has been demonstrated that leukocytes isolated from

the septic patients at this stage show an altered immunological status such as increased IL-10 secretion, anergy in T cells with a shift to Th2 phenotype, reduced MHC-II expression by antigen presenting cells.

In such patients, development of multi-organ failure and sepsis are seen as a frequent complication.

HOMEOSTATIS

ROLE OF INNATE IMMUNE CELLS IN SEPSIS NEUTROPHILS: At initial stages, phagocytic role of neutrophils assist in cleaning up

the bacterial load and reducing sepsis induced damage. In addition to antimicrobial components, during sepsis, following activation with PAMPs and pro-inflammatory cytokines, neutrophils express surface molecules such as CD64 (Receptor for IgG),CD14 which increases circulation.

One of the primary causes of intensified hyper-inflammatory stage is due to the suppression of apoptosis of circulating neutrophils, which is responsible for causing major tissue damages.

The possible causes of apoptosis suppression are attributed to sustained mitochondrial membrane potential and reduced caspase-9 activity andcaspase-8 phosphorylation.

MACROPHAGES: This cellular activation process leads to release of a large pool of pro-

inflammatory cytokines such as TNF-α, IL-6, IL-1β, IL-8, IL-12, platelet activating factor (PAF), ROS and microbicidal components (lysozyme, cationic proteins, acid hydrolases, and lactoferrin) that act against the microbes.

Recently, programmed cell death receptor (PD-1) was shown to be involved in the macrophage immune dysfunction. Research showed that deletion of PD1 inhibited migration of macrophages isolated from septic mice.

Endotoxins like LPS, macrophages enter into an unresponsive state and cannot respond to further challenge of endotoxin with a reduced pro-inflammatory cytokine and chemokine production and is termed as endotoxin tolerance in macrophages. As a result of endotoxin tolerance, there is a cellular reprogramming which up-regulates the expression of immunosuppressive mediators.

DENDRITIC CELLS: Dendritic Cells(DCs)function as the major antigen presenting cells for

T cell activation, secrete IL-12 and IL-10 and express MHCII and co-stimulatory molecules on their surfaces, thus acting as the connecting bridge between  innate and acquired immune systems.

According to a post-mortem study of patients with sepsis, both the number and area occupied by splenic DCs was highly reduced in trauma and sepsis patients as compared to healthy individuals.

Interaction of TLR2 and TLR4 on DCs with their respective ligands could induce depletion of DCs but the mechanisms behind apoptosis and deletion of DCs during sepsis remain unknown and requires further exploration.

ROLE OF ADAPTIVE IMMUNE CELLS IN SEPSIS T LYMPHOCYTES: The development and progression of sepsis is associated with a

dysfunction of innate and adaptive immune systems, (e.g. enhanced T and B cell apoptosis, inhibition of Th1 cell function, reduction in T cell receptor (TCR) function, and increased presence of T-regulatory cells in tissues.

The CD4+ T cells in sample tissues showed suppressed production of IFN-γ and TNF-α, decreased expression of CD127 (the IL-7R α-chain), but increased expression of PD1, which is another phenotypic feature of exhausted T cells.

The study showed increased its ligand, PDL1 expression in capillary endothelial cells and bronchial epithelial cells of septic patients potentially altering the function of T cells that have migrated to the site of infection. animal studies that showed inhibition of PD1–PDL1 interaction could improve survival in sepsis by reducing T cell exhaustion.

B CELLS: The role of B lymphocytes in the pathogenesis of sepsis has to be

explored.

There is a reduction in the peripheral blood B lymphocytes, which could be due to an increase in CD95 expression on the B cells as seen in septic shock patient.

Co-stimulatory molecules CD80 and CD86 which are critical molecules in the B-cell antigen presentation function, are also highly up-regulated in septic shock patients admitted to ICUs.

TREATMENT AVAILABLE To date, except recombinant activated protein C, FDA

has not approved any other drug for treating severe sepsis.

Recombinant human activated protein C which was the only FDA approved drug for treating sepsis was also shown to improve sepsis survival via inhibition of neutrophil chemotaxis via binding to neutrophils integrin’s.

Integrin α3β1 was up-regulated on a subset of neutrophils that showed hyper inflammatory phenotype during sepsis and deletion of this integrin from neutrophils improved survival of septic animals.

CONCLUSION

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