Gain-of-function Tibetan PHD2^D4E;C127S variant suppresses monocyte function: A lesson in inflammatory response to...

We have previously described an evolutionarily selected Tibetan prolyl hydroxylase-2 (PHD2^D4E;C127S) variant that degrades the hypoxia-inducible factor (HIFα) more efficiently and protects these highlanders from hypoxia-triggered elevation in haemoglobin concentration. High altitude is known to cause acute mountain sickness (AMS) and high-altitude pulmonary edema (HAPE) in a section of rapidly ascending non-acclimatised lowlanders. These morbidities are often accompanied by inflammatory response and exposure to hypobaric hypoxia is presumed to be the principal causative agent. We have investigated whether PHD2^D4E;C127S variant is associated with prevention of hypoxia-mediated inflammatory milieu in Tibetan highlanders and therefore identify a potential target to regulate inflammation.

We genotyped the Tibetans using DNA isolated from whole blood. Thereafter immunophenotying was performed on PBMCs from homozygous PHD2^D4E;C127S and PHD2^WT individuals using flow cytometry. RNA isolated from these individuals was used to evaluate the peripheral level of important transcripts associated with immune as well as hypoxia response employing the nCounter technology. The ex-vivo findings were validated by generating monocytic cell lines (U937 cell line) expressing PHD2^D4E;C127S and PHD2^WT variants post depletion of endogenous PHD2. We had also collected whole blood samples from healthy travellers and travellers afflicted with AMS and HAPE to evaluate the significance of our ex-vivo and in vitro findings. Hereafter, we also attempted to resolve hypoxia-triggered inflammation in vitro as well as in vivo by augmenting the function of PHD2 using alpha-ketoglutarate (αKG), a co-factor of PHD2.

We report that homozygous PHD2^D4E;C127S highlanders harbour less inflammatory and patrolling monocytes in circulation as compared to Tibetan PHD2^WT highlanders. In response to in vitro hypoxia, secretion of IL6 and IL1β from PHD2^D4E;C127S monocytes, and their chemotactic response compared to the PHD2^WT are compromised, corresponding to the down-modulated expression of related signalling molecules RELA, JUN, STAT1, ATF2 and CXCR4. We verified these functional outcomes in monocytic U937 cell line engineered to express PHD2^D4E;C127S and confirmed the down-modulation of the signalling molecules at protein level under hypoxia. In contrast, non-Tibetan sojourners with AMS and HAPE at high altitude (3,600 m above sea level) displayed significant increase in these inflammatory parameters. Our data henceforth underline the role of gain-of-function of PHD2 as the rate limiting factor to harness hyper-activation of monocytes in hypoxic environment. Therefore upon pre-treatment with αKG, we observed diminished inflammatory response of monocytes in vitro and reduction in leukocyte infiltration to the lungs in mice exposed to normobaric hypoxia.

Our report suggests that gain-of-function PHD2 ^D4E;C127S variant can therefore protect against inflammation elicited by hypobaric hypoxia. Augmentation of PHD2 activity therefore may be an important method to alleviate inflammatory response to inspired hypoxia.

This study is supported by the Department of Biotechnology, Government of India.