title: T-cell responses during Trypanosoma brucei infections
Creators: name: Millar, Amanda E.
Publisher: name: University of Glasgow
Year: 1997
Subjects: subject: Immunology, parasitology, Trypanosoma brucei, T-cells.

description: African trypanosomiasis is caused by Trypanosoma brucei and is a disease of considerable importance causing infection in both humans and livestock. There is a degree of protection from the antibody response produced by the host but it is ineffective and infections are chronic and debilitating. This chronicity is due to antigenic variation by the trypanosomes and to immunosuppression of the host. Antigenic variation is a classic, and well studied, B-cell evasion mechanism in a number of infections and can be observed at a highly sophisticated level during African trypanosome infections. As yet, it has been little studied as a possible T-cell evasion mechanism and this is the subject of my thesis. Initially, a reliable in vitro assay system was devised in order to examine the proliferative T-cell responses of mononuclear splenocyte populations as T-cell proliferation had not been detected during trypanosome infections in previous studies. Responses against trypanosome lysates, paraformaldehyde-fixed and live trypanosomes were examined with paraformaldehyde-fixed trypanosomes being the preferred choice of antigenic stimuli. The optimal conditions for this assay were also determined as far as mitogen concentration, concentration of trypanosome antigen and incubation time were concerned. Using this reliable proliferation assay system I examined a number of trypanosome infection and immunisation regimes. Most of these analyses were conducted using splenocytes taken from mice at first peak of parasitaemia employing parasite lines in each of which 99% of the trypanosomes present express the same variant antigen type (VAT) during the first parasitaemic wave. The mononuclear splenocytes from infected mice produced a high level of proliferation in response to mitogen stimulation but also produced trypanosome antigen-driven proliferation. This antigen-driven proliferative response was mainly against the homologous VATs but there was also a degree of heterologous antigen-driven proliferation in response to some VATs but not against others. Immunosuppression is considered to be a major contributing factor in continuation of an infection by dampening down any effective immune response during African trypanosomiasis. Immunosuppression is noticeable after the first peak of parasitaemia and involves a number of components with 'suppressor' macrophages appealing to play a key role. I therefore investigated the role of nitric oxide (NO), a major product of activated macrophages, in the regulation of T-cell responses during chronic Trypanosoma brucei infection. Using transgenic mice deficient in inducible nitric oxide synthase (iNOS), the proliferative and cytokine responses were examined. Daily parasitaemias were determined, nitrate levels calculated and antibody isotyping examined using infected and uninfected mice homozygous and heterozygous for the iNOS loci. This work highlighted the key role of NO in the regulation of T-cell responses during trypanosome infections: in the absence of iNOS activity there was an upregulation in proliferation, IFNgamma production and IL-2 receptor expression causing improved clearance of trypanosomes from the systemic circulation. In conclusion, this body of work has successfully designed an in vitro assay system to examine T-cell proliferation to trypanosome VATs and, using this assay, I have successfully detected trypanosome antigen-driven T-cell proliferation during acute infections. This proliferation was observed using mononuclear splenocyte populations from infected and immunised mice and was found to be homologous antigen-driven and of T-helper 1 type. There was however heterologous antigen-driven T-cell proliferation against some trypanosome VATs but a lack of proliferation against others. A pivotal role for NO in causing immunosuppression during chronic T. brucei infections was determined and this immunosuppression influenced the cytokine production and T-cell proliferative responses but did not appear to affect the antibody production. These data suggest that VAT-specific T-cell responses are an important component of the immune response to T. brucei. (Abstract shortened by ProQuest.).