Objective: Although acute hypoxia has significant pathophysiologic implications in health and disease, insight on its impact on both the innate and adaptive immune response is still limited. A pathophysiologic link between hypoxia and subsequent activation of immune components, including the complement system, appears existing. Here, we present a thorough examination of the impact of acute normobaric hypoxia on the regulation of the circulating complement system using quantitative mass spectrometry.

Methods: In a normobaric hypoxia chamber, hypoxia was generated by gradually reducing the partial pressure of oxygen in ambient air corresponding to that of high (6000 m;6k) altitude (pO2; 21.25 kPa (0k) and 9.64 kPa (6k)). The reoxygenation phase was included in the experiment after the induction of hypoxia, allowing for the evaluation of the effects of reoxygenation post hypoxia. We included in total 16 participants and measured multiple blood parameters as well as twelve plasma-associated components of the complement system via targeted quantitative mass spectrometry at the beginning, at 6k and at the reoxygenation phase. Each stage lasted two hours.

Results: Following hypoxia induction, the number of circulating leukocytes increased. Leukocytes continued to increase steadily after reaching peak hypoxia (6k) and reoxygenation. Neutrophils were the predominant population responsible for the hypoxia-induced increase. While lymphocyte number increased after reaching 6k, and dropped following re-oxygenation, monocytes frequency did not alter. After 6k, a considerable reduction in red blood cell width, which maintained after reoxygenation was observed. The platelet count increases dramatically after reaching simulated oxygen levels of 6k of elevation, but subsequently declines somewhat following reoxygenation (not significant). Hematocrit, cytokeratin, and mean corpuscular volume (MCV) levels all drop considerably from the start to the completion of the experiment. The erythrocyte count and mean corpuscular hemoglobin (MCH) levels remain steady. Creatinine levels decrease considerably after reaching 6k and continue to fall significantly after the trial is concluded. To determine hypoxia-dependent changes of the complement system, the C1 complex peptides (C1S, C1R, C1RL, C1QB, C1QC), the complement cascade (C3, C5, C7, C9), and three complement activation factors (CFAB, CFAH, CFAI) were investigated. After re-oxygenation, all complement peptides were significantly down-regulated relative to both the baseline and the hypoxic condition.

Conclusion:

Circulating levels of the major constituents of the complement system are rapidly reduced during hypoxia. These findings highlight hypoxia-related pro-inflammatory signalling in a number of systemic diseases, including high altitude-associated systemic inflammatory response.