1 THE EFFECT OF CO2 ON LPS-INDUCED CYTOKINE RESPONSES IN RAT ALVEOLAR MACROPHAGES Running Title: CO2 alters alveolar macrophage cytokine responses

Alveolar macrophages (AM) may be exposed to a range of CO2 and pH levels depending on their location in the alveoli and the health of the lung. Cytokines produced by AM contribute to inflammation in acute lung injury (ALI). Current ventilatory practices for the management of ALI favour low tidal volumes, which can give rise to increases in CO2 and changes in pH of the alveolar microenvironment. Here we examined the effect of CO2 on cytokine release from LPS-stimulated rat AM. AM were incubated for 1-4 h under different atmospheric gas mixtures ranging from 2.5-20 % CO2. To distinguish between effects of pH and CO2, the culture media was also buffered to pH 7.2 with NaHCO3. Cell metabolic activity, but not cell viability, decreased and increased significantly after 4 h at 20 % and 2.5 % CO2, respectively. Increasing CO2 decreased TNFsecretion but had no effect on lysate TNF-. Buffering the media abated the effects of CO2 on TNF- secretion. CO2 increased CINC-1 secretion only when the pH was buffered to 7.2. Effects of CO2 on cytokine responses were reversible. In conclusion, the effects of CO2 on cytokine lysate levels and/or secretion in AM are cytokine specific and depending on both the cytokine and the immediate microenvironment, may be beneficial or detrimental to ALI. 3