The goal of this project was to investigate the immunomodulatory and anti-tumour activity of aseries of novel monoclonal antibodies (mAbs) targeting human Glucocorticoid-induced TNFRrelated protein (GITR). GITR is a cell surface co-stimulatory receptor constitutively expressed at highlevels on T regulatory cells (Tregs) and at low levels on naïve and memory T cells. Its activationresults in the increased survival, proliferation and function of effector T cells as well as inhibitingthe suppressive capacity of Tregs. GITR is therefore an attractive target for anti-tumourimmunotherapies. To explore this, a panel of novel anti-human GITR mAbs were generated andinvestigated. To characterise these mAbs, both in-vitro and in-vivo methodologies were utilised.Surface plasmon resonance identified the binding affinities and binding domains of differentantibody clones. Human GITR (hGITR) expressing Jurkat/NF-κB/GFP reporter cells were generatedto understand how these mAbs influenced intracellular signalling pathways. Furthermore, CFSElabelled human peripheral blood mononuclear cells (PBMCs) were co-cultured with sub-optimalconcentrations of anti-hCD3, alongside anti-hGITR mAbs, to determine the ability of anti-hGITRmAbs to modulate human T cell proliferation in-vitro. To investigate these mAbs in-vivo, a novelhGITR knock-in (hGITRKI) transgenic mouse was developed and characterised, followed by studiesin mouse tumour models.The generated mAbs bound to either hGITR domain 1 or 2 with a range of affinities, with themajority increasing intracellular NF-κB. Two of the mAb clones reduced the proliferation of hCD3-stimulated T cells, demonstrating their ability to influence cellular function in human T cells in-vitro.However, this reduction was only observed when these clones were of the mIgG2a isotype, whichsuggested a potential mechanism of action through the mAb-mediated depletion of hGITRexpressing T cells. In-vivo impacts were explored in newly generated hGITRKI mice. Insertion of thechimeric hGITR gene into exon 1 of the mGITR gene resulted in the simultaneous expression of cellsurface hGITR as well as disruption of mGITR expression. Furthermore, hGITRKI mice displayed ahGITR expression pattern similar to that of human PBMCs. These mice were then used to determinethe therapeutic efficacy of anti-hGITR mAbs in-vivo. One of the anti-hGITR mAbs (84-9 mIgG2a) wasshown to reduce tumour size and increase survival in EG7-OVA tumour-bearing hGITRKI mice. Thistherapeutic benefit was dependent on antibody isotype. A potential mechanism of action was that84-9, in the mIgG2a format, depleted intratumoural Tregs which altered the ratio of effector toregulatory T cells within the tumour microenvironment. Future work will seek to further definemechanisms of action and the underlying properties that mediate its therapeutic activity incomparison to other mAbs directed to hGITR or other immune stimulatory receptors.