Subcutaneous Bacterial Challenge Is a Measure of Innate Immunity

To assess the effects of acute stress on innate immunity, we have completed a series of studies examining the effect of acute tailshock stress on the development and recovery from a subcutaneous challenge with live E. coli. The strain of bacteria and route of challenge were chosen for the following reasons: (1) the effect of subcutaneous E. coli in nonstressed rats has been thoroughly characterized. We (Campisi et al., 2003a) have previously completed dose-response experiments testing the effect of E. coli on measures of innate immune activation (peripheral and local cytokines) and sickness responses (fever and spontaneous activity). (2) Previous work (Deak et al., 1999) indicated that a subcutaneous injection of E. coli, rather than Staphy-lococcus aureus, stimulated a more consistent and robust in vivo inflammatory response. (3) E. coli challenge is known to stimulate an immediate innate inflammatory response (Campisi et al., 2002), and macrophages and neutrophils are primarily responsible for host defense to this pathogen (Ali et al., 1997; Fleshner et al., 2002). (4) E. coli is a common pathogen found in nature as well as in the laboratory. (5) E. coli is not normally present in subcutaneous tissue of nonchallenged animals. Thus, we can be confident that any E. coli retrieved and cultured for assessment of colony-forming units (cfu) is from the experimental challenge and not from endogenous bacteria. (6) The robust inflammatory response induced by subcutaneous injection of E. coli allows in vivo measurements of various features of the sickness response, such as the diameter and grade of inflammation (Deak et al., 1999; Campisi et al., 2002; Fleshner et al., 2002), fever (Campisi et al., 2003a), circulating and local cytokines (Campisi et al., 2003a), local NO (Campisi et al., 2003b), body weight loss (Campisi et al., 2002), and activity (Campisi et al., 2003a). (7) A subcutaneous injection allows site-specific analyses of the in vivo immune response using an ex vivo approach. That is, after exposure to in vivo stress and bacterial challenge, we can remove the inflammatory site, place it in culture, and assess the ongoing response by sampling culture supernatants. This is especially advantageous because assessment of peripheral circulating leukocytes and circulating cytokines has limited relevance to the local immune response generated in response to challenge.

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