Aims/hypothesis: Type 1 diabetes is the result of an inflammatory T helper 1 (Th1) lymphocyte-mediated beta cell destructive process. The majority of diabetesprone BioBreeding (BBdp) rats fed wheat protein-based diets, such as NTP-2000, develop type 1 diabetes and display a mild coeliac-like enteropathy. Mesenteric lymph nodes (MLNs), which drain the gut, are the major inductive site where dietary antigens are recognised in the gut-associated lymphoid tissue (GALT). We hypothesised that this compartment could be a site of abnormal wheat proteininduced Th1 cell activation. Methods: MLN cells were isolated from BBdp and BB control (BBc) rats that were fed NTP-2000 or a hydrolysed casein (HC)-based diet at ages that pre-date classic insulitis. The inflammatory status, phenotype and proliferation of these cells in response to wheat protein were determined. Results: The expression ratio of T-bet : Gata3, master transcription factors for Th1 and Th2 cytokines, was increased in the MLN from NTP-2000-fed BBdp rats compared with that from BBc rats, mainly due to decreased Gata3 expression. CD3 + CD4 + IFN-γ + T cells were more prevalent in the MLN of wheatfed BBdp rats, but remained at control levels in BBdp rats fed a diabetes-retardant HC diet. BBdp MLN cells proliferated in response to wheat protein antigens in a specific, dose-dependent manner, and >93% of cells were CD3 + CD4 + T cells. This proliferation was associated with a low proportion of CD4 + CD25 + T cells and a high proportion of dendritic cells in the MLN of BBdp rats. Conclusions/ interpretation: Before insulitis is established, the MLNs of wheat-fed BBdp rats contain an unusually high proportion of Th1 cells that proliferate specifically in response to wheat protein antigens.