Mast cells (MCs) are long-living multifunctional innate immune cells that originate from hematopoietic precursors and specifically differentiate in the destination tissue, e.g., skin, respiratory mucosa, intestine, where they mediate immune cell recruitment and antimicrobial defense. In vivo these tissues have characteristic physiological oxygen levels that are considerably lower than the atmospheric oxygen conditions (159 mmHg, 21% O2; 5% CO2) traditionally used to differentiate MCs and to study their functionality in vitro. Only little is known about the impact of physiological oxygen conditions on the differentiation process of MCs. This study aimed to characterize the differentiation of immature murine bone marrow-derived MCs under physioxia in vitro (7% O2; 53 mmHg; 5% CO2). Bone marrow-derived suspension cells were differentiated in the presence of interleukin-3 with continuous, non-invasive determination of the oxygen level using a Fibox4-PSt3 measurement system without technique-caused oxygen consumption. Trypan blue staining confirmed cellular viability during the specified period. Interestingly, MCs cultivated at 7% O2 showed a significantly delayed differentiation rate defined by CD117-positive cells, analyzed by flow cytometry, and reached >95% CD117 positive population at day 32 after isolation. Importantly, MCs differentiated under physioxia displayed a decreased transcript expression level of hif-1α and selected target genes vegf, il-6, and tnf-α, but an increase of foxo3 and vhl expression compared to MCs cultivated under normoxia. Moreover, the production of reactive oxygen species as well as the amount of intracellular stored histamine was significantly lower in MCs differentiated under low oxygen levels, which might have consequences for their function such as immunomodulation of other immune cells. These results show for the first time that physioxia substantially affect maturation and the properties of MCs and highlight the need to study their function under physiologically relevant oxygen conditions.

The obligate intracellular bacterium Chlamydia trachomatis inserts into the membrane of its vacuole (the inclusion) a family of poorly characterized Inc proteins. While the Inc CpoS was recently revealed as a critical suppressor of host cellular immune surveillance, the underlying mechanism remained unknown. By complementing a cpoS mutant with modified variants of CpoS, we found that CpoS blocks distinct cellular defense responses through distinct mechanisms. Specifically, we show that the ability of CpoS to interact with Rab GTPases is not only instrumental to its ability to mediate lipid transport to the inclusion, but also key to CpoS-mediated inhibition of type I interferon responses. Indeed, depletion of Rab35 can phenocopy the respective defect of the cpoS mutant. Unexpectedly, we found that CpoS is also essential for the formation of inclusion microdomains that control the spatial organization of multiple Incs involved in signaling and modulation of the host cellular cytoskeleton. Overall, our findings highlight the modulation of membrane trafficking as a pathogenic immune evasion strategy and the role of Inc-Inc interactions in shaping the inclusion microenvironment.