Effect of Advanced Age on the Innate Immune Response to Cutaneous Wound Infection
An estimated 25 billion in US health care expenditure is spent on care of chronic, non-healing wounds. The failure to effectively heal wounds is often compounded by co-morbidities, such as diabetes or obesity. Another major patient population afflicted with chronic wounds are the elderly. Advanced age is associated with a decline in immunologic function that contributes to a poor response to vaccination, infection and tissue injury resulting in prolonged hospital stays and age-related morbidity and mortality. Specifically, clinical observations and laboratory studies have suggested an age-related decline in cutaneous wound healing, marked by protracted wound closure, wound dehiscence and chronic wound states. Previous studies have suggested that age-related alterations in the early inflammatory phase of cutaneous wound injury may contribute to the lengthened and aberrant course of wound healing in the elderly. This impaired response to cutaneous injury offers a persistent portal of entry for foreign pathogens, such as Staphylococcus aureus (S. aureus), a common dermatopathogen that accounts for up to 50% of surgical site infections in the elderly. Underlying this elevated susceptibility to infection are deficits in host recognition, phagocytosis, migration and activation of a pathogen-specific adaptive immune response. Specific innate immune deficits have been tied to an elevated risk of S. aureus wound infection, including decreased neutrophil counts and function, reduced expression of cutaneous antimicrobial peptides and aberrant toll-like receptor 2 (TLR2) expression and/or signaling. However, the effect of age on these specific alterations in the innate response has not been well described in a model of cutaneous wound infection. Thus, we hypothesized that advanced age contributes to a functionally inept innate immune response that impairs resolution of S. aureus cutaneous wound infection. To elucidate the effects of advanced age on cutaneous wound infection, the specific aims were (i) To establish a model of local cutaneous wound infection in young and aged mice, (ii) To determine if age-related changes in leukocyte phagocytosis, leukocyte recruitment or cutaneous antimicrobial peptide expression contribute to age-associated differences in bacterial colonization and wound closure (iii)To determine if modulation of these age-associated deficits in the early innate response to S. aureus cutaneous wound infection may provide new therapeutic avenues for treating wound infection and promoting wound closure in the elderly. Aim 1 was to establish of model of cutaneous injury and S. aureus infection in young (3-4 month) and aged (18-20 month) BALB/c mice. Following injury and infection at days 1-10, we observed heightened bacterial colonization and delayed wound closure in aged animals as compared to young. In Aim 2, we utilized this model to evaluate keratinocyte and resident leukocyte TLR expression, leukocyte infiltration and phagocytosis and wound antimicrobial peptide expression. While no differences in infiltrating leukocyte TLR expression, phagocytosis or antimicrobial peptide expression were noted, decreased neutrophil and macrophage recruitment to the wound site were observed in aged mice relative to young. In an in vivo chemotaxis assay, we confirmed a reduced migratory capacity to increasing doses of the neutrophil chemokine KC in aged mice. Given these findings, we sought to modulate neutrophil recruitment to the wound site in Aim 3. Following cutaneous injury and infection, mice either received 3 s.c. injections of 250 pg of G-CSF or saline vehicle. G-CSF treatment in aged mice reduced bacterial colonization and wound size relative to saline controls. This was associated with increased neutrophil accumulation in wounds of aged, G-CSF treated mice at days 3 and 7 following injury and infection. While future work needs to be conducted to elucidate the mechanism by which G-CSF exerts these age-dependent effects, our data highlights a putative indication for G-CSF treatment within the wound care setting in the elderly patient population as well as patients with local or systemic neutropenia.
Aging, Anti-Microbial Peptides, Chemotaxis, Innate Immunology, Macrophage, Neutrophils, Cell and Developmental Biology
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