The Dermis as a Delivery Site of Trypanosoma brucei for Tsetse Flies

Abstract

Tsetse flies are the sole vectors of Trypanosoma brucei parasites that cause sleeping sickness. Our knowledge on the early interface between the infective metacyclic forms and the mammalian host skin is currently highly limited. Glossina morsitans flies infected with fluorescently tagged T. brucei parasites were used in this study to initiate natural infections in mice. Metacyclic trypanosomes were found to be highly infectious through the intradermal route in sharp contrast with blood stream form trypanosomes. Parasite emigration from the dermal inoculation site resulted in detectable parasite levels in the draining lymph nodes within 18 hours and in the peripheral blood within 42 h. A subset of parasites remained and actively proliferated in the dermis. By initiating mixed infections with differentially labeled parasites, dermal parasites were unequivocally shown to arise from the initial inoculum and not from a re-invasion from the blood circulation. Scanning electron microscopy demonstrated intricate interactions of these skin-residing parasites with adipocytes in the connective tissue, entanglement by reticular fibers of the periadipocytic baskets and embedment between collagen bundles. Experimental transmission experiments combined with molecular parasite detection in blood fed flies provided evidence that dermal trypanosomes can be acquired from the inoculation site immediately after the initial transmission. High resolution thermographic imaging also revealed that intradermal parasite expansion induces elevated skin surface temperatures. Collectively, the dermis represents a delivery site of the highly infective metacyclic trypanosomes from which the host is systemically colonized and where a proliferative subpopulation remains that is physically constrained by intricate interactions with adipocytes and collagen fibrous structures.

Tsetse fly tolerance to T. brucei infection: transcriptome analysis of trypanosome-associated changes in the tsetse fly salivary gland

“…For the infection experiment the pleiomorphic Trypanosoma brucei brucei ( Tbb ) AnTAR1, derived from the EATRO 1125 strain [8] was used. This tsetse-trypanosome infection model has already been shown to result in good infection rates in the fly midgut and salivary glands, with high metacyclic parasite densities in the latter [2, 9]. Freshly emerged flies were offered their first blood meal on an anaesthetized mouse showing a parasitaemia of approximately 10 8 trypanosomes/ml blood with 80% intermediate/stumpy forms.…”

Adipose Tissue: A Safe Haven for Parasites?

“…recently showed by quantitative PCR that the AT harbors around 10-fold more parasites than the average number found in the blood in chronic stages of the disease [20]. Interestingly, in the two studies describing tropism for the skin, many parasites were found in the vicinity of subcutaneous adipocytes [17,18]. Although it remains to be compared the relative number of parasites in skin, visceral adipose tissue and testis from the same mouse, there is no doubt that the adipose tissue is one of the major parasite reservoirs.…”

“…Replicating parasites have been found both in the AT and in the skin, suggesting these tissue environments provide enough nutrients to support parasite growth [17,18,20]. A large number of non-dividing transmissible forms have also been found in the two tissues [15], suggesting that transmission can happen from extra-vascular sites [17,20].…”

“…Live imaging revealed that indeed some T. brucei slender parasites present a fast persistent movement, while others are less mobile [17]. Conversely by scanning electron microscopy, parasites appear to tightly interact with subcutaneous adipocytes, with entanglement by reticular fibers and embedment between collagen bundles [17,18,20]. It remains unknown if the parasites interact with adipocytes to propel themselves forward, or as a means of slowing down at least temporarily.…”

Intravital microscopy: Imaging host–parasite interactions in lymphoid organs

“…Furthermore, not all LNs are equally accessible, and some have been preferred for intravital studies in various fields – in particular the popliteal LN. Plasmodium and Leishmania IVM imaging have been mostly done in the popliteal LN (Bajénoff et al, ; Radtke et al, ); ex vivo imaging of T. brucei infected was done in the cervical and mandibular LN (Caljon et al, ), while T. gondii was imaged by IVM in the mesenteric LN (Chtanova et al, ). Due to the vast possibilities for imaging LNs in situ by IVM, this section will focus on the technical details of visualization of the popliteal LN (Fig.…”