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Apolipoprotein A-IV (apoA-IV) is a 46-kDa exchangeable plasma apolipoprotein, secreted primarily by the intestinal mucosa and to a lesser extent by hepatocytes in humans and in most animals. Since its first description in 1974, a variety of functions has been proposed for apoA-IV, including involvement in the assembly, secretion, and metabolism of lipids, in the regulation of food intake as a satiety factor and modulator of gastric acid secretion, and in the protection against inflammation and atherosclerosis due to its antioxidant properties [1 and references therein]. ApoA-IV is synthesized in villous enterocytes (VE) and assembled to pre-chylomicron transport vesicles in the Golgi complex. The latter contributes to the maturation of pre-chylomicron particles along their vectorial transport to the basolateral surface, prior to secretion into the pericellular spaces adjacent to lymphatic fenestrae [2]. Despite considerable effort, the precise cellular function of apoAIV in enterocytes has not yet been elucidated. The follicle-associated epithelium (FAE) is a poorly characterized region of the intestinal mucosa that overlies mucosa-associated lymphoid tissue (e.g., Peyer’s patches). The FAE is clearly different from VE, since there is a high density of M (microfold) cells, specialized for antigen sampling and transcytosis of IgA, parallel with the lack goblet cells and subepithelial myofibroblasts [3 and references therein]. It is unclear whether FAE is comprised in part of predetermined M cell precursors, since the mRNA profile of FAE differs from the transcriptome of M cells or transcriptome of VE cells [3]. The FAE is considered as a major component of mucosal immune response. In this issue of Digestive Diseases and Sciences, Tokuhara et al. [4] provide evidence that apoA-IV mRNA and protein are constitutively expressed in cells of the FAE, but absent in M cells in the mouse small intestine. Moreover, mice strongly express apoA-IV protein in the jejunal VE before weaning, but surprisingly not in chowfed young (3-week-old) or adult (7-week-old) mice [4]. In addition to protein expression in the FAE, apoA-IV mRNA is expressed in jejunal VE during development before weaning, which continuously declines after weaning [4]. In adult mice, the mRNA expression of apoA-IV is largely restricted to the FAE and to the tips of jejunal VE [4]. A monoclonal antibody generated by the authors against mouse FAE recognized apoA-IV with high specificity [4], serving as a valuable and unique tool for apoA-IV protein localization, which had not been reported previously, due to lack of antibody specificity [1, 4 and references therein]. Considering the recent findings of Tokuhara et al. [4], the amount of apoA-IV expression in adult mouse VE is most likely very low in the basal or fasting state, with rapid induction following lipid ingestion, consistent with apoAIV expression in the VE of suckling mice [4], and with the extensive regulation of apoA-IV by nutritional and hormonal factors such as the fat in breast milk [1 and references therein]. Constitutive, age-independent expression of apoA-IV mRNA and protein in the mouse FAE is an important novel finding of Tokuhara et al. [4], adding a further element to the characterization of apoA-IV and to adjacent non-M cells in the FAE. The expression of apoA-IV is not mandatory for the organogenesis of Peyer’s patches, as apoAIV-deficient mice (global knockout) have morphologically intact Peyer’s patches [4] and develop no major E. Orso (&) G. Schmitz Institute for Laboratory Medicine and Transfusion Medicine, University Hospital Regensburg, Franz-Josef-Strauss-Allee 11, 93053 Regensburg, Germany e-mail: evelyn.orso@klinik.uni-regensburg.de

Apolipoprotein A-IV in the Follicle-Associated Epithelium: A Further Piece in the Puzzle