Gut Lymphocyte Phenotype Changes After Parenteral Nutrition and Neuropeptide Administration.

Parenteral Nutrition (PN) prevents progressive malnutrition in patients unable to take adequate nutrition via the GI tract. However, compared to enteral nutrition (EN), PN increases the risk of pneumonia and intra-abdominal abscesses, especially pneumonia, in severely injured trauma patients.1–5 The reason for this impairment with PN is multi-factorial but experimental evidence implicates an impairment of mucosal immunity. The main strategic adaptive component of the mucosal immune system is immunoglobulin A (IgA), a molecule which functions to prevent attachment of bacteria to mucosal surfaces and as well as control intra-luminal bacterial populations. The integrity of mucosal immunity and levels of IgA in both intestinal fluid and the lung depends upon enteral stimulation and feeding. 6–8 When compared to EN, PN with decreased enteral stimulation (PN/DES) in mice decreases both the absolute number of mucosal immune T & B cells and levels of IgA by 50–60% in both the lung and the gut. 6 These changes occur due to effects on T & B cell distribution, T & B cell phenotypes, chemokines, Th2-type cytokines, IgA production and IgA transport as listed in Table 1. 6–12 These effects with PN destroy established antibacterial and antiviral respiratory immunity in the mouse. 11, 13, 14 The relevance of the murine finding to the human condition have been strengthened through comparisons of intestinal immunity changes after PN 15 and in airway responses of the of mice and humans after injury. These experimental observations provide a cogent explanation for the increased risk of infectious in lungs of PN-fed patients. Table 1 Alterations in mucosal immunity associated with parenteral nutrition The enteric nervous system (ENS) appears to be intimately associated with the mucosal immune system. The ENS forms a vast network of neurons throughout the gastrointestinal tract with an estimated 3 m of nerve per cm3 of gastrointestinal tissue with most fibers located within 13 micrometers of the mucosa. Neuropeptides synthesized by the ENS regulate gut motility and secretion, mucosal growth and immune function defenses. 16–20 One neuropeptide released in humans soon after ingestion of food is gastrin-releasing peptide (GRP). GRP shares an identical 7-amino acid carboxyl terminus to bombesin (BBS), a neuropeptide originally isolated from the skin of the frog Bombina bombina. BBS is frequently used to study GRP function due to their similar receptor interactions. 16 BBS (and GRP) stimulates the release of many gastrointestinal hormones including gastrin, CCK, and neurotensin. 16, 18 Our lab previously characterized the effects of BBS supplementation with PN (PN+BBS) on mucosal immune defenses. In our model, BBS preserves lymphocyte cell mass in the gut associated lymphoid tissue (GALT), including T & B lymphocytes in Peyer’s patches and the lamina propria and restores mucosal IgA levels in the lungs and the gut. 13, 21–23 BBS also restores established antibacterial and antiviral activity lost during PN. 11, 21 PN/DES reduces the absolute number of T & B cells in the lamina propria, lung and Peyer’s patches with a reduction of CD4+ cells and a reduced CD4/CD8 ration in the lamina propria. Detailed changes of specific lymphocyte phenotypes in mucosal immune sites such as those listed in Table 2 are lacking. In this work we hypothesized that PN/DES decreases lymphocyte phenotypes important in homing, activation, immunologic memory and IgA production. In addition, we hypothesized that the administration of BBS during PN treatment reverses these phenotype changes. We believe that by studying the effect of PN/DES on mucosal immunity and the role of the enteric nervous system in regulating this process allows better understanding mucosal immunity changes associated with injury and critical care management. Since some patients inevitably have prolonged contraindications to EN and require PN to avoid progressive protein-calorie malnutrition, this work may allow the development of therapies which correct these deficits and alleviate some infectious complications associated with PN/DES. Table 2 Lymphocyte Surface Markers with Corresponding Phenotypes and Descriptions MATERIALS AND METHODS Animals Male six-to-eight-week-old Institute of Cancer Research mice were purchased from Harlan (Indianapolis, IN) and housed in the Animal Research Facility of the William S. Middleton Memorial Veterans Hospital, an American Association for Accreditation of Laboratory Animal Care accredited conventional facility. Mice were allowed to acclimatize for 1 week with free access to standard chow diet (PMI Nutritional International, St. Louis, MO) and water, under controlled conditions of temperature and humidity with a 12:12 hour light:dark cycle.