oa Editorial [Hot Topics: Mucosal Immunity: From the Eye Surface to the Gut (Guest Editor: Horacio Marcelo Serra)]

Mucosal surfaces such as those in the ocular surface, oral cavity, and gut are protected by a network of organized structures known as the Common Mucosal Immune System. These mucosal associated lymphoid tissues (MALT) include different glands, the Waldeyer's ring, Peyers patches and isolated lymphoid follicles. Several excellent articles have recently reviewed various aspects of these specialized tissues therefore, I have selected for this hot topic, contributions that highlight recent advances in closely related yet distinct fields of research. In the first article Li et al. focus on the superficial ocular environment describing the roles of defensins in combating ocular infection and in the modulation of inflammation. They also examine the biological activities and regulated expression of defensins in corneal and conjunctival epithelial cells. Echegaray et al. review the participation of corneal epithelium in the regulation of the ocular immune responses and describe how the corneal epithelial cells have the ability to respond to innate immune responses throughout the whole corneal tissue and regulate the recruitment of immunological cells in a way that corneal clarity is maintained. Then Valentich et al. highlight advances in the recent knowledge about the anatomy/histology of the oral cavity and the associated immunological structures trying to envisage what may happen when antigens enter in the mouth. Oral mucosa has received attention in the last decade because it offers excellent accessibility and avoids degradation of proteins and peptides. Although the moist lining tissue of the oral cavity, the oral mucosa, is continuous with the remaining gastrointestinal tract, structurally the oral mucosa has more in common with skin than with the gastrointestinal mucosa. The review of Correa et al. deals with the cross talk between intestinal epithelial cells (IECs) and leukocytes. The network of dendritic cells (DCs) in the vicinity of IECs is crucial in this process as DC maturation across the epithelial barrier seems to be dependent on the presence of specific surface factors. IECs are broadly unresponsive to gram-positive bacterial components, notably TLR-2 ligands, in contrast to gram-negative bacterial components. The TLR signalling is tightly regulated in IECs to avoid uncontrolled inflammation by several negative regulators. The nature and the species of micro flora acquired in the first few months of life depends on many factors including, external environmental micro flora, use of antibiotics, immunomodulatory agents, and breast or artificial feeding. Mucosal epithelial cells and Paneth cells produce a variety of effectors molecules that protect mucosal surfaces against invading microbes. The colonizing microbes participate in the host development and homeostasis as well. Finally Lira et al. discuss expression and function of chemokines in the intestine trying to clarify the picture of the role of chemokines in trafficking of leukocytes in the different areas of this organ.