Glycerol metabolism triggers trypanosome differentiation into transmissible forms in mammalian tissue-like conditions

In the mammalian bloodstream, Trypanosoma brucei, the parasite responsible for sleeping sickness, proliferates as slender forms before undergoing quorum-sensing (QS)-mediated differentiation into cell cycle-arrested stumpy forms (stumpy-QS), a transition that regulates parasitaemia and primes parasites for tsetse fly transmission. Beyond the bloodstream, T. brucei also occupies extravascular, adipose-rich tissues such as the skin, a potential reservoir for transmission. Here, we identify an alternative slender-to-stumpy differentiation pathway driven by glycerol, a host metabolite abundant in adipose-rich tissues, that could resolve the long-standing paradox of successful parasite transmission from human hosts during chronic infection despite low parasitaemia. We show that high (10 mM) and non-physiological glycerol concentrations under low glucose conditions (0.5 mM) induce differentiation, generating distinct stumpy-Glyc forms that resemble stumpy-QS parasites but have an extended lifespan. Under conditions mimicking dermal tissue interstitial fluids (4 mM glucose, 0.25 mM glycerol), we show that glycerol promotes the emergence of proliferative intermediate forms that retain transmission potential and can differentiate into fly host-specific procyclic forms in vitro and within tsetse flies. These findings open the door for reevaluation of the model of T. brucei transmission and supports a dominant role for adipocyte-derived glycerol in the skin in sustaining parasite transmission.