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Primary immunodeficiency syndromes (PID) are rare disorders caused by genetic defects affecting development and function of adaptive and/or innate immunity. The most frequent presenting feature of PID in children is infection. However, only approximately 10% of children referred with recurrent infections have a demonstrable immunodeficiency. Management of children affected by PIDs includes supportive and corrective treatment. Supportive measures focus on prevention and treatment of infection; non-infective complications may require anti-inflammatory and/or immunosuppressive treatment. All children affected by primary immunodeficiency should be monitored regularly in a specialist paediatric immunology centre. The immunophenotype can predict the likely underlying molecular defect. Serum immunoglobulin levels may be helpful although they are variable in young infants and confounded by the presence of maternal IgG in infants less than 6 months of age. Inflammatory skin disease may require immunosuppression with steroids and/or ciclosporin. Wiskott–Aldrich syndrome typically consists of the triad of thrombocytopenia, atypical eczema and immunological abnormalities.

Peripheral blood mononuclear cells (PBMCs) proliferate inconsistently, rendering current lymphoproliferation assays unreliable in diagnosis.To investigate the utility and nature of proliferation responses in allergy by comparison of the standard lymphoproliferation with a new dendritic cell (DC) stimulated assay.Monocyte-derived DCs were pulsed with allergens and incubated with autologous T cells for 7 days. DC-stimulated and standard PBMC proliferation responses to 3 common dietary allergens in children with allergy and without atopy were measured by incorporation of tritiated thymidine and reduction of carboxyl fluorescein succinimidyl ester staining.The DC presentation of sensitizing allergens induced significantly higher proliferative responses than PBMC stimulation (P = .04) and greater distinction between normal and allergic responses. DC-induced stimulation indices of children without sensitivity and those with allergy were significantly different with all 3 foods (P.001). All children with allergy presented with peanut allergy and 12 of 14 (86%) β-lactoglobulin-pulsed DC preparations proliferated more than 3.3-fold above un-pulsed cells, but 8 of 18 children (44%) with ovalbumin egg allergy showed proliferation below this level. The stimulation index of DC tritiated thymidine incorporation correlated closely with carboxyl fluorescein succinimidyl ester reduction (P .001). Sensitivity of detection of peanut, milk, or egg allergy was 100%, 85.7%, or 55.6% and specificity was 60%, 88.9%, or 86.7%, respectively. DC-stimulated T cells expressed increased levels of CD45 RO and CD25 and most produced interferon-γ. DC-stimulated proliferation correlated with total immunoglobulin E and peanut antigen-stimulated proliferation correlated with peanut specific immunoglobulin E (P = .03).The DC-induced lymphoproliferation had higher sensitivity, specificity, and reproducibility than the standard assay and caused increased memory and activated T-cell proliferation in children with food allergy.

Scott-Taylor TH, Hourihane J, Strobel S. Correlation of allergen-specific IgG subclass antibodies and T lymphocyte cytokine responses in children with multiple food allergies. Pediatr Allergy Immunol 2010: 21: 935–944. © 2010 John Wiley & Sons A/S Cytokines can affect the quantity and class of allergen-specific immunoglobulins through the T cell polarization that accompanies atopy. Antigen-specific IgG subclasses and IgE antibodies were compared with intracellular T cell cytokine changes to sensitizing antigens in 23 children with multiple food allergies and 20 healthy controls. Allergic children showed higher levels of total and food-specific IgE, IgG1 and IgG4 to peanut, milk and egg than non-atopic children or adults, coinciding with a TH2 cytokine response to sensitizing antigens. IgG1 and IgG4 antibodies specific to milk and egg and peanut protein were elevated relative to age-matched healthy children (p ≤ 0.05) and, in milk- and egg-sensitized children, correlated with cytokine responses (p

In contrast to healthy conditions, intestinal epithelial cells (IECs) stimulate proinflammatory CD4+and CD8+T cells during Crohn's disease (CD). The underlying regulatory mechanisms remain unknown. Here we investigated the epithelial expression of major histocompatibility complex (MHC) I and MHC II and its interference with endocytic pathways, in vivo. During ileoscopy, ovalbumin (OVA) was sprayed onto ileal mucosa of CD patients (ileitis and remission) and controls. The epithelial traffic of OVA and MHC I/II pathways were studied in biopsies using fluorescence and electron microscopy. We found MHC I and MHC II to accumulate within multivesicular late endosomes (MVLE) of IECs. Faint labeling for these molecules was seen in early endosomes and lysosomes. MVLE were entered by OVA 10 min after exposure. Exosomes carrying MHC I, MHC II, and OVA were detected in intercellular spaces of the epithelium. OVA trafficking and labeling patterns for MHC I and MHC II in IECs showed no differences between CD patients and controls. Independent of inflammatory stimuli, MHC I and MHC II pathways intersect MVLE in IECs, which were efficiently targeted by luminal antigens. Similar to MHC II-enriched compartments in professional antigen presenting cells, these MVLE might be critically involved in MHC I- and MHC II-related antigen processing in IECs and the source of epithelial-released exosomes. The access of luminal antigens to MHC I in MVLE might indicate that the presentation of exogenous antigens by IECs must not be restricted to MHC II but might also occur as “cross-presentation” via MHC I to CD8+T cells.

Hemophagocytic lymphohistiocytosis (HLH) composes a group of rare heterogenous disorders characterized by uncontrolled accumulation and infiltration of activated T lymphocytes and macrophages. Cytotoxic T and natural killer cell activity is significantly reduced or absent in these patients. Mutations in the important mediator of lymphocyte cytotoxicity perforin were identified in a number of HLH individuals. Here we report a novel missense mutation thr435met in the conserved Ca(2+) binding domain of perforin in a patient with HLH. Prediction of the 3-dimensional structure of the thr435met perforin mutant using comparative molecular modeling indicates that the protein's ability to bind Ca(2+), and therefore its cytolytic function, would be strongly compromised. In addition, this patient exhibited abnormal CD45 splicing caused by a C77G mutation in the gene encoding CD45 (PTPRC). Our findings suggest a combined role for perforin mutation and abnormal CD45 splicing as significant contributory factors in the pathogenesis of HLH.

Assessment with questionnaire, skin tests, and immunoglobulin E measurements showed 6 (0.5%) of 1263 children having elective surgery had latex allergy; 50 (4%) others were latex-sensitized. Any previous operation increased the odds of latex sensitization by 13 times; multiple operations had a less marked effect. All children having surgery may need primary prophylaxis for latex.

Summary The intestinal tract is the major immunological organ of the human body. The gut associated lymphoid tissues (GALT) play an important role in the suppression of adverse reactions to foods and in the induction of “Oral Tolerance”. Immunologically mediated clinical reactions to foods exist outside the gut environment and can affect virtually all organ systems.

Heterozygous mutations in neutrophil elastase have been detected in many sporadic cases of congenital neutropenia. However, a convincing pathogenetic mechanism has not been established, and it is unclear whether the effects of the mutant enzyme occur within the cell of production or are paracrine in nature. The healthy father of a patient was demonstrated to be mosaic for his daughter's Cys42Arg elastase mutation. Using semiquantitative polymerase chain reaction, approximately half of his T cells were shown to carry the mutation in contrast to less than 10% of neutrophils. Individual hematopoietic colonies grown from peripheral blood were heterozygous for the mutation or were homozygous wild type. These results demonstrate that precursors containing the mutation are selectively lost during myelopoiesis or fail to develop into neutrophils. This is the first in vivo confirmation of the pathogenic nature of elastase mutations in humans. The normal neutrophil count in the father suggests that the mutant elastase does not have paracrine effects.

Cytokines can affect the quantity and class of allergen-specific immunoglobulins through the T cell polarization that accompanies atopy. Antigen-specific IgG subclasses and IgE antibodies were compared with intracellular T cell cytokine changes to sensitizing antigens in 23 children with multiple food allergies and 20 healthy controls. Allergic children showed higher levels of total and food-specific IgE, IgG1 and IgG4 to peanut, milk and egg than non-atopic children or adults, coinciding with a TH2 cytokine response to sensitizing antigens. IgG1 and IgG4 antibodies specific to milk and egg and peanut protein were elevated relative to age-matched healthy children (por= 0.05) and, in milk- and egg-sensitized children, correlated with cytokine responses (p0.05). Peanut-sensitized children additionally had elevated levels of IgG2 and IgG3 also which correlated inversely (p0.003 and p0.04, respectively) with IFNgamma production. Elevated allergen-specific IgG subclass antibodies in sensitized children correlated with total IgE levels (por= 0.05) in all three food allergen groups. The ratio of specific IgG1 to IgG4 was highest in those with high IgE, inverted with resolution of allergy, and correlated with total IgE levels (por= 0.01) in milk- and egg-sensitized children. The correlation of TH2 responses with allergen-specific antibodies would implicate polarized T cells in food allergic children in IgE hypersensitivity and overproduction of particular IgG subclasses alike. IgG1:IgG4 ratio declines with allergy sensitization and may denote emerging tolerance.

Clinical experience ("the dangerous bottle") and experimental evidence indicate that the early life of an infant is particularly important for the development of the immune responses to food antigens. However, the clinical and immunologic consequences of a brief exposure to, or avoidance of, food antigens during the neonatal period in human infants are poorly understood and documented. We present the preliminary results of a prospective controlled study of 256 normal breast-fed infants randomly assigned to receive (blind) either an adapted formula (Nidina) or a partially hydrolyzed formula (Nidal HA) as a supplement to breast-feeding for a few days when necessary, and to be examined at days 5, 90, 150, and 365. The results indicated that (1) the prevalence of clinical symptoms and of total and specific IgE responses was not statistically different in the two groups of infants and (2) infants fed a hydrolyzed formula had median titers of specific IgG lower than those fed an adapted formula; the difference was significant for alpha-lactalbumin at day 90 (p0.005) and for alpha-lactalbumin (p0.05), casein (p0.05), and hydrolyzed formula (p0.01) at day 150. Humoral immune responses of breast-fed infants to food antigens thus appear to be modulated by early, short-term exposure to them.

SUMMARY Following an application from Cambridge Theranostics Ltd. submitted pursuant to Article 14, of Regulation (EC) No 1924/2006 via the Competent Authority of United Kingdom, the Panel on Dietetic Products, Nutrition and Allergies was asked to deliver an opinion on the scientific substantiation of a health claim related to lycopene-whey complex (bioavailable lycopene) and reduction of the risk of atherosclerotic plaques. The scope of the application was proposed to fall under a health claim referring to disease risk reduction. The food constituent that is the subject of the health claim is a lycopene-whey complex (bioavailable lycopene). Lycopene-whey complex is prepared using a patented process by mixing tomato extract containing 10% lycopene with whey protein to form a granulated product. The amount of the active constituent, lycopene, is 2% in the final product. The lycopene-whey complex has been shown to have bioavailability similar to tomato paste in human subjects.

SUMMARY Following a request from the European Commission, the Panel on Dietetic Products, Nutrition and Allergies was asked to carry out the additional assessment for ‘Glucosamine Hydrochloride from Aspergillus niger’ as a food ingredient in the context of Regulation (EC) No. 258/97 taking account of the comments/objections of a scientific nature raised by the Member States. The applicant intends to add glucosamine hydrochloride from the fungus Aspergillus niger (RGHAN) to fruit juices, fruit “smoothies”, dehydrated instant drink mixes, fermented milkbased products, sports drinks and iced tea drinks, at a concentration that would provide approximately 750 mg per daily serving. The target consumers would be older people and sportsmen and sportswomen. The RGHAN production process is similar to that used to isolate glucosamine hydrochloride from shellfish except that the source of the raw material is the biomass from an A. niger fermentation. A. niger has been used for citric acid production since the 1920s. The strain of A. niger used in this process is not genetically-modified, pathogenic or toxic and does not produce the mycotoxin ochratoxin A. The chitin-containing biomass is hydrolysed using food-grade concentrated hydrochloric acid at 100° C to form glucosamine hydrochloride, which is then concentrated by filtration of the biomass and evaporative crystallisation of the filtrate to yield glucosamine hydrochloride crystals. These are centrifuged, washed with water and flash dried. The product RGHAN meets the United States Pharmacopoeia-National Formulary (USP-NF) specification for glucosamine hydrochloride of > 98 %, loss on drying of < 1 %, arsenic < 3 ppm and heavy metals < 0.001 %. Analyses of five non-consecutive

SUMMARY Following an application from Mead Johnson & Company submitted pursuant to Article 14 of Regulation (EC) No 1924/2006 via the Competent Authority of France, the Panel on Dietetic Products, Nutrition and Allergies was asked to deliver an opinion on the scientific substantiation of a health claim related to docosahexaenoic acid and arachidonic acid and brain development. The scope of the application was proposed to fall under a health claim referring to children’s development and health. The food constituents that are the subject of the health claim are docosahexaenoic acid (DHA) and arachidonic acid (ARA), which are well characterised fatty acids that can be quantified in foods by established methods. The absorption of DHA and ARA is well documented. The Panel considers that the food constituents DHA and ARA are sufficiently characterised. The claimed effect is the contribution to the optimal brain development of infants and young children. The target population proposed by the applicant is infants and young children (from birth to three years of age). The Panel considers that contribution to the normal development of the brain is beneficial for infants’ and children’s development and health. The applicant identified a total 33 publications as being pertinent to the health claim for humans. A total of 13 full publications which report original data from RCTs on the effects of 1 For citation purposes: Scientific Opinion of the Panel on Dietetic Products, Nutrition and Allergies on a request from Mead

This review discusses experimental and clinical evidence for the existence of immunologically mediated damage to the intestinal mucosa. The discussion of mechanisms of gastrointestinal immunoregulation and lymphocyte recirculation via recently described homing receptors is preceded by a summary of the basic structures of the gut-associated lymphoid tissues (GALT). The relevance of food-induced systemic hyporesponsiveness (oral tolerance) to the prevention of immunologically mediated damage to intestinal mucosa is discussed in the light of anaphylactic (Type I) and delayed type (Type IV) hypersensitivity reactions. Other, less well defined, mechanisms of allergic injury are also briefly discussed. It is postulated that the abrogation and/or modulation of orally induced tolerance is the pathogenetic mechanism underlying the induction of food-sensitive enteropathies.

Epicutaneous immunization is a potential non-invasive technique for antigen-specific immune-modulation. Topical application of protein antigens to barrier-disrupted skin induces potent antigen-specific immunity with a strong Th2-bias. In this study, we investigate whether the autoimmune inflammatory response of chronic collagen-induced arthritis (CCIA) in DBA/1-TCR-beta Tg mice can be modified by epicutaneous immunization. We show that epicutaneous immunization with type II collagen (CII) inhibited development and progression of CCIA and, importantly, also ameliorated ongoing disease as indicated by clinical scores of disease severity, paw swelling and joints histology. Treated mice show reduced CII-driven T cell proliferation and IFN-gamma production, as well as significantly lower levels of CII-specific IgG2a serum antibodies. In contrast, CII-driven IL-4 production and IgE antibody levels were increased consistent with skewing of the CII response from Th1 to Th2 in treated mice. IL-4 production in treated mice was inversely correlated with disease severity. Moreover, T cells from treated mice inhibited proliferation and IFN-gamma production by T cells from CCIA mice, suggesting induction of regulatory T cells that actively inhibit effector responses in arthritic mice. The levels of CD4(+)CD25(+) T cells were however not increased following epicutaneous CII treatment. Together, these results suggest that epicutaneous immunization may be used as an immune-modulating procedure to actively re-programme pathogenic Th1 responses, and could have potential as a novel specific and simple treatment for chronic autoimmune inflammatory diseases such as rheumatoid arthritis.

Epicutaneous immunization is a potential novel technique for topical vaccine delivery. It targets the immunologically rich milieu of the skin while having the advantage of being a non-invasive immunization procedure. By disrupting the stratum corneum of the epidermis a natural adjuvant effect can be achieved through activation of resident Langerhans cells. This negates the normal need for co-application of noxious adjuvants. Epicutaneous immunization on barrier-disrupted skin induces potent antigen-specific systemic immunity with a strong T helper type 2 (Th2) bias. We show here that epicutaneous immunization enhances the vigour of a subsequent T-cell response to the same antigen. The induced systemic Th2 response prevents the development of Th1 responses induced through injection of antigen in complete Freund's adjuvant (CFA). Prior epicutaneous immunization results in reduced production of antigen-specific interferon-gamma and immunoglobulin G2a (IgG2a) and enhanced interleukin-4, IgG1 and IgE responses to immunization with CFA. Moreover, epicutaneous immunization converts an established Th1 response to a Th2 response, as demonstrated by the specific reduction of interferon-gamma and IgG2a and the enhancement of interleukin-4 and IgE. This Th2 dominance of epicutaneous immunization may have direct therapeutic application as an immune-modulating procedure in Th1-dominant diseases such as autoimmune rheumatoid arthritis, type 1 diabetes, Hashimoto's thyroiditis and multiple sclerosis.

In Crohn's disease (CD), colonic epithelial cells (CECs) are suggested to stimulate pro-inflammatory CD4+ T cells. However, the endocytic pathways of luminal antigens involved in underlying MHC class II presentation by CECs remain unknown. Our aim was to elucidate antigen trafficking and associated MHC class II expression in CECs of CD patients in vivo. In CD patients (Crohn's colitis and remission) and healthy controls undergoing colonoscopy, ovalbumin (OVA) was sprayed onto inflamed or healthy mucosa. The subcellular localization of OVA and MHC class II was visualized in biopsies taken from OVA-incubated mucosa using fluorescence and cryoelectron microscopy. Targeting of OVA into late endosomes of CECs was found in healthy (controls and CD in remission) and inflamed mucosa (Crohn's colitis). MHC class II expression in CECs was not detected in healthy mucosa but strongly up-regulated during CD inflammation. Induced MHC class II in CECs was predominantly seen at basolateral membranes and in late endosomes, which were efficiently accessed by internalized OVA. Our data provide in vivo evidence that the endocytic pathway of luminal antigens in CECs of Crohn's colitis patients intersects MHC class II-enriched late endosomes and support the postulated role of CECs in MHC class II-associated antigen presentation during CD.

Most allergic, atopic and hypersensitive reactions are associated with Th2-biased immune responses and allergen-specific IgE antibodies. Pathological allergic disorders are on an alarming increase in the industrialized world. Understanding the mechanism of primary sensitization to allergens is important in elucidating the pathogenesis of these diseases and for possibly preventing their development. In this article, we review recent information supporting that epidermal allergen exposure may contribute to systemic allergic diseases and that atopy may be secondary to skin barrier dysfunction in some dermatoses. The skin is an active immunological organ, which functions as a primary defence and biosensor to the external environment. The critical permeability barrier function is mediated by the outmost layer of the epidermis, the stratum corneum. Perturbation of the stratum corneum initiates a chain of event, which activates homeostatic responses in the underlying epidermis. Repeated barrier-disruption, whether environmentally or genetically determined, may however stimulate signaling cascades that lead to inflammation and epidermal hyperplasia. Skin barrier dysfunction may also allow entry of allergens, which can lead to primary systemic sensitization. The altered epidermal microenvironment in barrier-disrupted skin appears to be particularly well suited for the induction of potent Th2-type responses with production of allergen-specific IgE. Epidermal exposure to food antigens can prevent the normal induction of oral tolerance and also lead to airway eosinophilia following inhalation. Exposure to allergens on barrier-disrupted skin may as such serve as a natural sensitization pathway for food allergy and respiratory allergic disease.

The prevalence of food allergic diseases is rising and poses an increasing clinical problem. Peanut allergy affects around 1% of the population and is a common food allergy associated with severe clinical manifestations. The exact route of primary sensitization is unknown although the gastrointestinal immune system is likely to play an important role. Exposure of the gastrointestinal tract to soluble antigens normally leads to a state of antigen-specific systemic hyporesponsiveness (oral tolerance). A deviation from this process is thought to be responsible for food-allergic diseases. In this study, we have developed a murine model to investigate immunoregulatory processes after ingestion of peanut protein and compared this to a model of oral tolerance to chicken egg ovalbumin (OVA). We demonstrate that oral tolerance induction is highly dose dependent and differs for the allergenic proteins peanut and OVA. Tolerance to peanut requires a significantly higher oral dose than tolerance to OVA. Low doses of peanut are more likely to induce oral sensitization and increased production of interleukin-4 and specific immunoglobulin E upon challenge. When tolerance is induced both T helper 1 and 2 responses are suppressed. These results show that oral tolerance to peanut can be induced experimentally but that peanut proteins have a potent sensitizing effect. This model can now be used to define regulatory mechanisms following oral exposure to allergenic proteins on local, mucosal and systemic immunity and to investigate the immunomodulating effects of non-oral routes of allergen exposure on the development of allergic sensitization to peanut and other food allergens.

The skin is an important immunological organ with an outer protective layer, the stratum corneum forming a barrier between the skin-associated lymphoid tissue and the environment. We show that gently removing the stratum corneum with adhesive tape permits potent epicutaneous immunization to protein antigens. IL-4 secretion by T cells from draining lymph nodes and high levels of specific IgE and IgG1 with no IgG2a showed that the immune responses induced following epicutaneous antigen exposure are strongly Th2 biased. Similar responses were obtained with different antigens and mouse strains. In contrast, subcutaneous immunization with antigen delivery into the dermis was less potent and gave predominantly Th1 responses. Removal of the stratum corneum increased expression of MHC class II, CD86, CD40, CD54 and CD11c on Langerhans cells, but did not cause them to migrate. Rapid migration from epidermis to draining lymph node was obtained, however, by exposure to antigen after removal of the stratum corneum, suggesting that maturation and migration of Langerhans cells are independently regulated events. These results suggest that antigen presentation by Langerhans cells gives predominantly Th2 responses. This may provide an explanation for allergic sensitization to some antigens. It may also be a useful non-invasive, non-adjuvant-dependent method of vaccination.

Hemophagocytic lymphohistiocytosis (HLH) and Langerhans cell histiocytosis (LCH) are members of a group of rare heterogenous disorders, the histiocytoses, characterized by uncontrolled accumulation of pleomorphic infiltrates of leukocytes. The etiology of these diseases is mainly unknown. CD45 is a hemopoietic cell specific tyrosine phosphatase essential for antigen receptor mediated signaling in lymphocytes and different patterns of CD45 splicing are associated with distinct functions. Recently a polymorphism (C77G) in exon 4 of CD45 causing abnormal CD45 splicing and a point mutation affecting CD45 dimerization were implicated in multiple sclerosis in humans and lymphoproliferation and autoimmunity in mice respectively. Here we show that two patients with HLH exhibited abnormal CD45 splicing caused by the C77G variant allele, while a further 21 HLH patients have normal CD45. We have also examined 62 LCH patients and found three to have the C77G mutation. Peripheral blood thymus-derived (T) CD8(+) cells from normal individuals carrying the C77G mutation show a significant decrease in the proportion of cells expressing L-selectin and increased frequency of cells with LFA-1(hi) expression. It remains to be established whether C77G is a contributing factor in these histiocytic disorders.

Convincing clinical and experimental evidence suggests that the disturbance of important immunoregulatory and suppressive immunological events induced after oral (mucosal) antigen exposure (oral tolerance) may lead to allergic and autoimmune diseases. Within a variety of factors, age of the host and timing of antigen (food) administration are important characteristics in the development of food allergic disease. Induction of tolerance is seen as a Th2 skewed response, which on one side may prevent harmful mucosal immune reactions but on the other side may contribute to adverse responses in the susceptible individual. The primary mechanisms by which tolerance may be mediated include deletion, anergy, suppression, "ignorance," and apoptosis. Cell-mediated delayed hypersensitivity reactions (Th1), which are implicated in the development of autoimmune and gastrointestinal diseases, are particularly well suppressed. Regulatory events after mucosal exposure of antigen are not well characterized and remain controversial. The balance between tolerance (suppression) and sensitization (priming) is dependent on several factors, such as: (a) genetic background, (b) nature and dose of antigen, (c) frequency of administration, (d) age at first antigen exposure, (e) immunological status of the host, (f) antigen transmission via breast milk, and others. Overall there is evidence in rodents that multiple low-dose feeds are more likely to induce regulatory cytokines (e.g., TGF-beta, IL-10, IL-4) in part secreted by CD4+CD25+ T regulatory cells. Despite the powerful suppressive effects of oral autoantigen exposure observed in experimental models of autoimmune diseases (including bystander suppression), their translation into clinical trials of autoimmune diseases has not yet yielded the expected beneficial results.

Editor,—We are concerned that the general readership of Archives of Disease in Childhood may be misled by a paper based on results of non-standardised or obsolete tests used in the diagnosis of children with potentially severe allergies to peanut and tree nuts.1 The implicit confidence of the paper's title is not supported by the experimental approaches. We would like to express our serious concern about the following specific points. It cannot be assumed a nut is “the only possible allergen” in a composite food. Any allergenic food can cross contaminate a “safe food.” Intradermal testing is not considered standard practice …

Background & Aims: Oral tolerance is recognized as a central immunoregulatory phenomenon. The mechanisms of its induction remain unclear, and the role of the intestinal epithelial cells that are able to present antigens to T lymphocytes is poorly understood. In this report, we analyze under in vivo conditions the intracellular targeting of mucosally administered ovalbumin (OVA) to major histocompatibility complex (MHC) class II antigen containing compartments of enterocytes and compare these pathways between BALB/c and SCID mice, the latter being unable to generate a transferable tolerogenic moiety after a feed of OVA. Methods: OVA, lysosome-associated membrane proteins (LAMP-1), and MHC class II antigens were localized in jejunal biopsy specimens of BALB/c and SCID mice at 0, 5, 10, 20, 40, 60, and 120 minutes after a single feed with OVA by fluorescence and electron microscopy. Results: Ten minutes after oral administration, OVA was transported to the proximity of MHC class II antigens within LAMP-1–positive vacuoles and to the basolateral membrane of enterocytes from BALB/c strain mice. However, in SCID mice, OVA reached the paracellular spaces during the same time period through LAMP-1–negative vacuoles of enterocytes, which lacked MHC class II antigens. Conclusions: Orally administered OVA is rapidly targeted to late endosomes containing LAMP-1 and MHC class II antigens in enterocytes of BALB/c mice but not in SCID mice bred on a BALB/c background. We suggest that this targeting process within the enterocytes is one of the requirements for the induction of oral tolerance. GASTROENTEROLOGY 2000;118:128-137