Maturation of a Trypanosoma brucei infection to the infectious metacyclic stage is enhanced in nutritionally stressed tsetse flies

We report on the effect of tsetse fly starvation on the maturation of an established Trypanosoma brucei brucei midgut infection, i.e., the development of procyclic infection into the infectious metacyclic parasites in the tsetse fly salivary glands. Glossina morsitans morsitans flies were nutritionally stressed 10 d after the uptake of a T. b. brucei-infected bloodmeal by depriving these flies from feeding for seven consecutive days, whereas the control fly group (nonstarved group) continued to be fed three times a week. After this period, both fly groups were again fed three times per week on uninfected rabbit. Thirty days after the infected bloodmeal, all surviving flies were dissected and examined for the presence of an immature midgut and a mature salivary gland trypanosome infections. Results showed a significantly increased proportion of flies with salivary gland infection in the nutritionally stressed fly group suggesting an enhanced maturation of the trypanosome infection. These data suggest that environmental factors that cause nutritional stress in a tsetse population do not only make tsetse flies significantly more susceptible to establish a midgut infection as was shown previously but also boost the maturation of these midgut infections.

Nutritional stress affects the tsetse fly's immune gene expression

Tsetse-transmitted trypanosomiasis poses a serious threat to human and animal health in sub-Saharan Africa. The majority of tsetse flies (Glossina spp.) in a natural population will not develop a mature infection of either Trypanosoma congolense or Trypanosoma brucei sp. because of refractoriness, a phenomenon that is affected by different factors, including the tsetse fly's immune defence. Starvation of tsetse flies significantly increases their susceptibility to the establishment of a trypanosome infection. This paper reports the effects of nutritional stress (starvation) on (a) uninduced baseline levels of gene expression of the antimicrobial peptides attacin, defensin and cecropin in the tsetse fly, and (b) levels of expression induced in response to bacterial (Escherichia coli) or trypanosomal challenge. In newly emerged, unfed tsetse flies, starvation significantly lowers baseline levels of antimicrobial peptide gene expression, especially for attacin and cecropin. In response to trypanosome challenge, only non-starved older flies showed a significant increase in antimicrobial peptide gene expression within 5 days of ingestion of a trypanosome-containing bloodmeal, especially with T. brucei bloodstream forms. These data suggest that a decreased expression of immune genes in newly hatched flies or a lack of immune responsiveness to trypanosomes in older flies, both occurring as a result of fly starvation, may be among the factors contributing to the increased susceptibility of nutritionally stressed tsetse flies to trypanosome infection.

Ability of trypanosome-infected tsetse flies (Diptera: Glossinidae) to acquire an infection with a second trypanosome species

The epidemiology of human and animal trypanosomiasis is determined to a large extent by the number of infected tsetse flies in a specific area. In the field, a substantial proportion of infected flies carry mixed trypanosome infections. The way in which these tsetse flies acquire a mixed infection is not fully understood. In particular, the susceptibility of tsetse flies to sequential infection with trypanosomes is not well understood. Accordingly, laboratory studies were made of the effects of age and prior infection on the probability of Glossina morsitans morsitans (Westwood) developing an infection of Trypanosoma congolense and Trypanosoma brucei brucei after feeding on infected mice. Results of these experiments clearly showed that 20-30-d-old G. m. morsitans can still pick up and develop a mature infection in the mouthparts/hypopharynx for T. congolense or in the salivary glands for T. b. brucei. However, their ability to acquire infection was significantly lower compared with teneral flies. Furthermore, 20-30-d-old flies that already carry a mature T. congolense or T. b. brucei infection remained at least as susceptible to a secondary trypanosome infection compared with noninfected flies of the same age. The immunological and epidemiological repercussions of those findings are discussed.

A role for the dynamic acylation of a cluster of cysteine residues in regulating the activity of the glycosylphosphatidylinositol-specific phospholipase C of Trypanosoma brucei

The glycosylphosphatidylinositol-specific phospholipase C or VSG lipase is the enzyme responsible for the cleavage of the glycosylphosphatidylinositol anchor of the variant surface glycoprotein (VSG) and concomitant release of the surface coat in Trypanosoma brucei during osmotic shock or extracellular acidic stress. In Xenopus laevis oocytes the VSG lipase was expressed as a nonacylated and a thioacylated form. This thioacylation occurred within a cluster of three cysteine residues but was not essential for catalytic activity per se. These two forms were also detected in trypanosomes and appeared to be present at roughly equivalent amounts. A reversible shift to the acylated form occurred when cells were triggered to release the VSG by either nonlytic acid stress or osmotic lysis. A wild type VSG lipase or a gene mutated in the three codons for the acylated cysteines were reinserted in the genome of a trypanosome null mutant for this gene. A comparative analysis of these revertant trypanosomes indicated that thioacylation might be involved in regulating enzyme access to the VSG substrate.