Helmerhorst EJ. Despite sequence homologies to gluten, salivary proline-rich proteins do not elicit immune responses central to the pathogenesis of celiac disease. Am J Physiol Gastrointest Liver Physiol 309: G910 -G917, 2015. First published October 1, 2015; doi:10.1152/ajpgi.00157.2015.-Celiac disease (CD) is an inflammatory disorder triggered by ingested gluten, causing immunemediated damage to the small-intestinal mucosa. Gluten proteins are strikingly similar in amino acid composition and sequence to prolinerich proteins (PRPs) in human saliva. On the basis of this feature and their shared destination in the gastrointestinal tract, we hypothesized that salivary PRPs may modulate gluten-mediated immune responses in CD. Parotid salivary secretions were collected from CD patients, refractory CD patients, non-CD patients with functional gastrointestinal complaints, and healthy controls. Structural similarities of PRPs with gluten were probed with anti-gliadin antibodies. Immune responses to PRPs were investigated toward CD patient-derived peripheral blood mononuclear cells and in a humanized transgenic HLA-DQ2/DQ8 mouse model for CD. Anti-gliadin antibodies weakly cross-reacted with the abundant salivary amylase but not with PRPs. Likewise, the R5 antibody, recognizing potential antigenic gluten epitopes, showed negligible reactivity to salivary proteins from all groups. Inflammatory responses in peripheral blood mononuclear cells were provoked by gliadins whereas responses to PRPs were similar to control levels, and PRPs did not compete with gliadins in immune stimulation. In vivo, PRP peptides were well tolerated and nonimmunogenic in the transgenic HLA-DQ2/DQ8 mouse model. Collectively, although structurally similar to dietary gluten, salivary PRPs were nonimmunogenic in CD patients and in a transgenic HLA-DQ2/DQ8 mouse model for CD. It is possible that salivary PRPs play a role in tolerance induction to gluten early in life. Deciphering the structural basis for the lack of immunogenicity of salivary PRPs may further our understanding of the toxicity of gluten. celiac disease; immune response; salivary protein; gluten; mouse model CELIAC DISEASE (CD) is a T cell-mediated inflammatory enteropathy that manifests itself in genetically predisposed individuals. In its etiology, both genetic and environmental factors are implicated. The major genetic risk factors are carriage of HLA-DQ2 heterodimers, expressed in 90 -95% of CD patients, or HLA-DQ8 heterodimers, expressed in the remainder of CD patients. The primary environmental cause of CD is ingested gluten, a heterogeneous mixture of glutamine-and proline-rich storage proteins present in wheat, rye, and barley (10,12,22,46). According to solubility in alcoholic solutions, gluten is divided into soluble gliadins and insoluble glutenins. Gluten proteins are unusual because they have a high content of proline (P) and glutamine (Q). In the most abundant gliadins, Q comprises 35-38% of the total amino acids, while P ranges from 13-17% (49, 62, 63). The high P conten...