Wound trauma mediated inflammatory signaling attenuates a tissue regenerative response in MRL/MpJ mice

Journal of Inflammation RWeseoaruchnd trauma mediated inflammatory signaling attenuates a tissue regenerative response in MRL/MpJ mice Stephen R Zins 0 Mihret F Amare 0 Khairul Anam 0 Eric A Elster 0 1 Thomas A Davis 0 0 Regenerative Medicine Department, Operational and Undersea Medicine Directorate at the Naval Medical Research Center Silver Spring , MD 20910- 7500 , USA 1 Department of Surgery, Uniformed Services University of the Health Sciences , Bethesda, MD 20814 , USA Background: Severe trauma can induce pathophysiological responses that have marked inflammatory components. The development of systemic inflammation following severe thermal injury has been implicated in immune dysfunction, delayed wound healing, multi-system organ failure and increased mortality. Methods: In this study, we examined the impact of thermal injury-induced systemic inflammation on the healing response of a secondary wound in the MRL/MpJ mouse model, which was anatomically remote from the primary site of trauma, a wound that typically undergoes scarless healing in this specific strain. Ear-hole wounds in MRL/MpJ mice have previously displayed accelerated healing and tissue regeneration in the absence of a secondary insult. Results: Severe thermal injury in addition to distal ear-hole wounds induced marked local and systemic inflammatory responses in the lungs and significantly augmented the expression of inflammatory mediators in the ear tissue. By day 14, 61% of the ear-hole wounds from thermally injured mice demonstrated extensive inflammation with marked inflammatory cell infiltration, extensive ulceration, and various level of necrosis to the point where a large percentage (38%) had to be euthanized early during the study due to extensive necrosis, inflammation and ear deformation. By day 35, ear-hole wounds in mice not subjected to thermal injury were completely closed, while the ear-hole wounds in thermally injured mice exhibited less inflammation and necrosis and only closed partially (62%). Thermal injury resulted in marked increases in serum levels of IL-6, TNF, KC (CXCL1), and MIP-2 (CXCL2). Interestingly, attenuated early ear wound healing in the thermally injured mouse resulted in incomplete tissue regeneration in addition to a marked inflammatory response, as evidenced by the histological appearance of the wound and increased transcription of potent inflammatory mediators. Conclusion: These findings suggest that the observed systemic inflammatory response of a severe thermal injury undoubtedly has an adverse effect on wound healing and tissue regeneration. - Background Wound healing is a complex process involving many cell types and mediators that regulate tissue repair. Successful wound healing and tissue regeneration depends on tightly regulated hemostasis, inflammation, matrix synthesis, proliferation, wound contraction and tissue remodeling to restore tissue function and integrity [1,2]. A thermal injury is among the most severe forms of trauma with effects both locally and systemically [3]. Patients with large burn injuries have a multitude of immunological alterations and impaired functions of multiple effector cells of innate immunity and acquired immunity (including macrophages, dendritic cells (DC), natural killer (NK) cells, and T cells) at the wound site or a systemic change in circulating inflammatory mediators [3-8]. Systemic inflammation can lead to profound suppression of the innate and adaptive immune system [4-8] resulting in increased sepsis, wound healing complications, multi-system organ failure, and remote organ injury at sites such as the lung, liver, small intestines, and brain, representing major causes of morbidity and mortality in burn trauma patients [3,9]. These thermally induced organ injuries appear to be caused by toxic inflammatory mediators produced by infiltrating activated neutrophils early after thermal injury that are associated with increased chemokine levels [10-13]. The complex balance between innate and adaptive immune cell function after a severe injury is vital in determining wound healing outcome [4]. Innate immune cells show a progressive increase in the production of pro-inflammatory immune regulatory molecules (IL-1, IL-6, TNF and PGE2), while cells of the adaptive immune system display counter-inflammatory responses such as IL-10 and TGF [13-15]. The interplay between pro- and anti-inflammatory mechanisms is key for avoiding further tissue damage beyond that of the primary insult and a systemic inflammatory response [4,68,13,16]. Mice of the MRL/MpJ strain have been reported to have a unique capacity for limited regenerative wound healing, as shown by the complete closure of 2-mm earhole wounds [17-19]. Excised tissue is quickly replaced with normal tissue architecture that retains its full functionality. In contrast, others have shown that small, open, excisional cutaneous wounds in MRL/MpJ mice heal with marked scarring and no evidence of tissue regeneration [17,20-22]. Recently, our laboratory reported that a severe thermal wound on the dorsum of MRL/MpJ mice heal with scar formation and a delay in two critical wound healing events: wound closure and myofibroblast development [22]. The mechanism(s) involved are unclear, but it appears that the anatomical site of the injury, the severity of the injury, and the milieu of pro- or anti-inflammatory cytokines are all critical factors in determining whether a wound heals with or without a scar [20,21,23-25]. Moreover, in the MRL/MpJ mouse model we have demonstrated that the systemic response to a severe thermal injury can trigger a lethal autoimmune response within weeks-to-months following severe traumatic injury [26]. Understanding the dichotomous role of innate immune responses and inflammation on tissue regeneration versus delayed healing and scar formation may ultimately lead to innovative approaches for treatment of severe wounds to promote accelerated and scarless healing as well as tissue regeneration. In this study we show in wild-type MRL/MpJ mice that scarless ear-hole healing does not occur following a severe thermal injury at an anatomically remote site. During the early inflammatory phases of healing, we observed marked pathological cutaneous skin lesions on the ear pinnae, including hyperkeratosis, acanthosis, mononuclear cell infiltration and necrosis in close proximity to ear-hole wound margins. In addition, we observed a significantly augmented inflammatory response in the serum, lung, ear wound, and burn wound margin tissue by analyzing various chemokine/cytokine expression levels. These findings underscore the profound importance of the systemic inflammatory response following peripheral tissue injury which can modulate other cellular events critical in wound healing, as evidenced by the impediment of an otherwise normal and complete wound healing-tissue regenerative response in MRL/MpJ mice. Age matched (8-12 weeks) female MRL/MpJ mice were purchase (...truncated)