African trypanosomes (Trypanosoma brucei sp.) are single-celled extracellular
protozoan parasites that are transmitted via the tsetse fly vector across sub-
Saharan Africa. T. brucei subspecies cause trypanosomiasis in both humans
and animals, inflicting substantial disease and economic strains in affected
regions. Mammalian infection begins when the tsetse fly takes a blood meal
and injects trypanosomes into the dermal layer of skin. The parasites then
invade the circulatory and lymphatic systems, reaching the draining lymph
nodes and disseminate systemically. Little is understood about the host-pathogen
interactions which influence the establishment of host infection at
the initial bite site in the skin. Most experimental transmissions of African
trypanosomiasis have studied the intraperitoneal or intravenous routes of
exposure. However, these by-pass the natural route of infection via the skin.
Therefore the aim of this thesis is to investigate the pathogenesis of African
trypanosome infection via the skin.
Chemokines play important roles in attracting leukocytes towards the
lymphatics and lymph nodes. To investigate how trypanosomes migrate from
the bite site to the draining lymph nodes, the hypothesis that chemokines may
act as chemoattractants for trypanosomes was tested. Chemokines can also
possess antimicrobial properties, including against the related protozoan
parasite Leishmania mexicana, therefore their potential cytotoxic effects
against T. brucei were tested. Data presented in this thesis shows that these
chemokines do not induce the chemotaxis of T. brucei. The motility
characteristics of the parasites were also not affected by chemokine exposure.
Nor did these chemokines exert any trypanostatic effects on trypanosomes.
These data suggest trypanosomes use alternative cues to reach the
lymphatics post-infection.
The skin is an overlooked area of research for African trypanosome infections.
Therefore work in this thesis sought to investigate the hypothesis that the
infection kinetics would be different in a host infected by the natural intradermal
route when compared with the routinely-researched intraperitoneal route.
Experiments in this thesis revealed clear differences in the infection kinetics
and disease progression in mice infected intradermally when compared with
those infected by the intraperitoneal route. These data imply that further
infection models should utilise intradermal injections and investigate the
overlooked skin stage of disease which occurs naturally in the wild.
Upon deposition in the skin the trypanosomes home towards the lymphatic
system before migrating systemically. Lymphotoxin-β-receptor signalling
(LTβR) is essential for lymphoid organogenesis and the maintenance of
secondary lymphoid tissue microarchitecture. For example, LTβ-/- mice lack
most lymph nodes and have grossly disturbed splenic microarchitecture. As a
consequence of these disturbances LTβ-/- mice have impaired antibody isotype
class-switching. Experiments in this thesis were performed to test the
hypothesis that deficiencies in lymph node development and antibody isotype
class-switching would influence disease pathogenesis. These data show that
disease susceptibility and pathogenesis were exacerbated in LTβ-/- mice,
which lacked class-switched antibody isotypes in their sera. This disease
profile was then reversed in LTβ-/- mice which received wild-type bone-marrow
transfers after their haematopoietic system was ablated through lethal
irradiation. These data could identify the importance of the class-switching
capability of the adaptive immune system to combat trypanosome infection.
Little is known of the early host-parasite interactions following injection of T.
brucei into the dermis of the skin. Macrophages are key players in the innate
immune response against African trypanosome infection, and manipulating
these cells during infection may help our understanding of the disease
pathogenesis. To address their potential role in disease susceptibility,
experiments were designed to manipulate the density and inflammatory status
of the macrophages in the skin prior to infection with T. brucei. These data
show, that manipulation of the inflammatory status of the skin reduced
susceptibility to infection with T. brucei via the skin. A greater understanding
of the macrophage-parasite interactions which occur during the early stages
of African trypanosome infection is important for understanding how the
immune system responds to infection and how we can boost immunity to
combat infection.
A thorough identification of the mechanisms involved in establishing African
trypanosome infections in the skin and their systemic dissemination will aid the
development of novel approaches to block disease transmission
