Abstract: Specific allergen injection immunotherapy is highly effective in IgE-mediated diseases, such as allergic rhinitis and venom anaphylaxis. Immunotherapy inhibits both early and late responses to allergen exposure. Immunotherapy is accompanied by increases in allergen-specific IgG, particularly the IgG4 isotype, which blocks not only IgE-dependent histamine release from basophils but also IgE-mediated antigen presentation to T cells. Immunotherapy acts on T cells to modify peripheral and mucosal TH2 responses to allergen in favor of TH1 responses. Recent studies have identified increased IL-10 production in peripheral blood and mucosal surfaces after immunotherapy. IL-10 has numerous potential antiallergic properties, including suppression of mast cell, eosinophil, and T-cell responses, as well as acting on B cells to favor heavy chain class switching to IgG4. These IL-10–producing cells might be so-called regulatory T cells and appear to be identified by the CD4+CD25+ phenotype. Studies in mice suggest that dendritic cells play a vital role in induction of regulatory T cells. Novel approaches to immunotherapy currently being explored include the use of adjuvants, such as monophosphoryl lipid A or nucleotide immunostimulatory sequences derived from bacteria that potentiate TH1 responses. Alternative strategies include the use of allergen-derived peptides or modified recombinant allergen vaccines that act on T cells while minimizing the IgE-dependent mast cell activation that is dependent on the native allergen conformation. Specific allergen injection immunotherapy is highly effective in IgE-mediated diseases, such as allergic rhinitis and venom anaphylaxis. Immunotherapy inhibits both early and late responses to allergen exposure. Immunotherapy is accompanied by increases in allergen-specific IgG, particularly the IgG4 isotype, which blocks not only IgE-dependent histamine release from basophils but also IgE-mediated antigen presentation to T cells. Immunotherapy acts on T cells to modify peripheral and mucosal TH2 responses to allergen in favor of TH1 responses. Recent studies have identified increased IL-10 production in peripheral blood and mucosal surfaces after immunotherapy. IL-10 has numerous potential antiallergic properties, including suppression of mast cell, eosinophil, and T-cell responses, as well as acting on B cells to favor heavy chain class switching to IgG4. These IL-10–producing cells might be so-called regulatory T cells and appear to be identified by the CD4+CD25+ phenotype. Studies in mice suggest that dendritic cells play a vital role in induction of regulatory T cells. Novel approaches to immunotherapy currently being explored include the use of adjuvants, such as monophosphoryl lipid A or nucleotide immunostimulatory sequences derived from bacteria that potentiate TH1 responses. Alternative strategies include the use of allergen-derived peptides or modified recombinant allergen vaccines that act on T cells while minimizing the IgE-dependent mast cell activation that is dependent on the native allergen conformation. Allergen injection immunotherapy is highly effective in carefully selected patients with IgE-mediated disease and represents the only routinely administered antigen-specific immunomodulatory treatment given for immunologic disease of any kind. Immunotherapy has been shown to be effective for venom anaphylaxis and for rhinoconjunctivitis and asthma caused by inhalant allergens.1Bousquet J. Lockey R. Malling H.J. Allergen immunotherapy: therapeutic vaccines for allergic diseases. A WHO position paper.J Allergy Clin Immunol. 1998; 102: 558-562Abstract Full Text Full Text PDF PubMed Scopus (754) Google Scholar, 2Lockey R.F. "ARIA": global guidelines and new forms of allergen immunotherapy.J Allergy Clin Immunol. 2001; 108: 497-499Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar Immunotherapy is particularly effective in seasonal pollinosis.3Varney V.A. Gaga M. Frew A.J. Aber V.R. Kay A.B. Durham S.R. Usefulness of immunotherapy in patients with severe summer hay fever uncontrolled by antiallergic drugs.BMJ. 1991; 302: 265-269Crossref PubMed Scopus (364) Google Scholar Immunotherapy improves seasonal asthma, inhibits seasonal increases in bronchial hyperresponsiveness, and improves quality of life in patients with hay fever.4Walker S.M. Pajno G.B. Lima M.T. Wilson D.R. Durham S.R. Grass pollen immunotherapy for seasonal rhinitis and asthma: a randomized, controlled trial.J Allergy Clin Immunol. 2001; 107: 87-93Abstract Full Text PDF PubMed Scopus (257) Google Scholar Immunotherapy confers long-term benefit for at least 3 years after discontinuation.5Durham S.R. Walker S.M. Varga E.M. Jacobson M.R. O'Brien F. Noble W. et al.Long-term clinical efficacy of grass-pollen immunotherapy.N Engl J Med. 1999; 341: 468-475Crossref PubMed Scopus (1239) Google Scholar, 6Golden D.B. Kwiterovich K.A. Kagey-Sobotka A. Valentine M.D. Lichtenstein L.M. Discontinuing venom immunotherapy: outcome after five years.J Allergy Clin Immunol. 1996; 97: 579-587Abstract Full Text Full Text PDF PubMed Scopus (156) Google Scholar In children immunotherapy has been shown to prevent onset of new sensitizations7Pajno G.B. Barberio G. De Luca F. Morabito L. Parmiani S. Prevention of new sensitizations in asthmatic children monosensitized to house dust mite by specific immunotherapy. A six-year follow-up study.Clin Exp Allergy. 2001; 31: 1392-1397Crossref PubMed Scopus (536) Google Scholar and to reduce progression of rhinitis to physician-diagnosed asthma.8Moller C. Dreborg S. Ferdousi H.A. Halken S. Host A. Jacobsen L. et al.Pollen immunotherapy reduces the development of asthma in children with seasonal rhinoconjunctivitis (the PAT-study).J Allergy Clin Immunol. 2002; 109: 251-256Abstract Full Text Full Text PDF PubMed Scopus (987) Google Scholar Elucidation of mechanisms of immunotherapy has potential implications, not only for the refinement of allergy vaccines but also for the development of tolerogenic vaccines in other branches of medicine. The allergen specificity of allergic disease and immunotherapy also means that effects of the allergen exposure might be observed either during experimental provocation in a clinical laboratory or during controlled natural exposure. This is in contrast to other immune diseases in which the antigen might be unknown or endogenous. The natural history of an experimental allergen challenge to the nose, eyes, or bronchi is the immediate development of mast cell–dependent sneeze, itch, watery discharge, or wheeze–chest tightness that was maximal at 15 to 30 minutes and resolved within 1 to 3 hours. A proportion of subjects have a late allergic response that manifests in the nose as nasal obstruction and in the bronchi as airway obstruction. Classically, late responses are maximal at 6 to 12 hours and resolve within 24 hours. The mechanism is distinct from that of the immediate response, being characterized by the recruitment, activation, and persistence of eosinophils, basophils, and activated T cells at the sites of allergen exposure. The immunopathologic changes in the mucosal tissues of subjects chronically exposed to inhalant allergens resemble those seen during the late response. Cytokines produced by T cells probably play a major role in orchestrating allergic inflammation. THl cells produce IFN-γ and IL-2 but not IL-4 or IL-5 after activation. TH2 cells produce mainly IL-4, IL-13, and IL-5 but not IL-2 or IFN-γ. TH2 cells characterize human allergic responses and are present at mucosal surfaces during the late but not immediate response to allergen exposure. TH2 cells can also be expanded from peripheral blood of allergic patients by means of stimulation with specific allergen in vitro. The proallergic effects of IL-4, IL-13, and IL-5 produced by TH2 cells are numerous and described elsewhere. Factors that determine the evolution of either TH1 responses, TH2 responses, or both include the route and dose of antigen and the nature of the antigen-presenting cell. For example, high doses of allergen might preferentially favor the induction of TH1-type responses.9Secrist H. DeKruyff R.H. Umetsu D.T. Interleukin 4 production by CD4+ T cells from allergic individuals is modulated by antigen concentration and antigen-presenting cell type.J Exp Med. 1995; 181: 1081-1089Crossref PubMed Scopus (260) Google Scholar Antigen-presenting cell subsets might direct the development of TH1 and TH2 responses. For example, DC1 and DC2 dendritic cell subsets have been implicated in the development of TH1 and TH2 responses, respectively.10Kapsenberg M.L. Hilkens C.M. Wierenga E.A. Kalinski P. The paradigm of type 1 and type 2 antigen-presenting cells. Implications for atopic allergy.Clin Exp Allergy. 1999; 29: 33-36Crossref PubMed Scopus (117) Google Scholar DC2-type dendritic cells have been identified in atopic subjects,11Reider N. Reider D. Ebner S. Holzmann S. Herold M. Fritsch P. et al.Dendritic cells contribute to the development of atopy by an insufficiency in IL-12 production.J Allergy Clin Immunol. 2002; 109: 89-95Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar and their ability to drive TH2 responses appears to relate to low levels of IL-12 expression. Early research into immunotherapy mechanisms examined circulating antibody responses. More recently, the focus has been on T-cell responses. Most work has examined the effect of subcutaneous immunotherapy rather than immunotherapy by alternative routes. Mechanisms might have a degree of heterogeneity, reflecting variability in the types of allergic diseases treated, the different patient populations, the use of different adjuvants, and the route, dose, and duration of treatment. In conventional pollen immunotherapy, serum IgE concentrations show little response,12Gehlhar K. Schlaak M. Becker W. Bufe A. Monitoring allergen immunotherapy of pollen-allergic patients: the ratio of allergen-specific IgG4 to IgG1 correlates with clinical outcome.Clin Exp Allergy. 1999; 29: 497-506Crossref PubMed Scopus (134) Google Scholar although seasonal increases in IgE are blunted.13Lichtenstein L.M. Ishizaka K. Norman P.S. Sobotka A.K. Hill B.M. IgE antibody measurements in ragweed hay fever. Relationship to clinical severity and the results of immunotherapy.J Clin Invest. 1973; 52: 472-482Crossref PubMed Scopus (298) Google Scholar A potential untoward effect of immunotherapy is the emergence of novel IgE responses to hitherto unrecognized allergenic components of the extract used for treatment,14Moverare R. Elfman L. Vesterinen E. Metso T. Haahtela T. Development of new IgE specificities to allergenic components in birch pollen extract during specific immunotherapy studied with immunoblotting and Pharmacia CAP System.Allergy. 2002; 57: 423-430Crossref PubMed Scopus (158) Google Scholar although the clinical significance of this phenomenon is unknown. Immunotherapy with inhalant allergens is associated with increases in serum allergen-specific IgG1, IgG4, and IgA levels.12Gehlhar K. Schlaak M. Becker W. Bufe A. Monitoring allergen immunotherapy of pollen-allergic patients: the ratio of allergen-specific IgG4 to IgG1 correlates with clinical outcome.Clin Exp Allergy. 1999; 29: 497-506Crossref PubMed Scopus (134) Google Scholar, 15Jutel M. Akdis M. Budak F. Aebischer-Casaulta C. Wrzyszcz M. Blaser K. et al.IL-10 and TGF-beta cooperate in the regulatory T cell response to mucosal allergens in normal immunity and specific immunotherapy.Eur J Immunol. 2003; 33: 1205-1214Crossref PubMed Scopus (794) Google Scholar In dust mite and venom immunotherapy, specific IgG4 responses are detectable within 60 days of starting treatment.15Jutel M. Akdis M. Budak F. Aebischer-Casaulta C. Wrzyszcz M. Blaser K. et al.IL-10 and TGF-beta cooperate in the regulatory T cell response to mucosal allergens in normal immunity and specific immunotherapy.Eur J Immunol. 2003; 33: 1205-1214Crossref PubMed Scopus (794) Google Scholar, 16Muller U. Akdis C.A. Fricker M. Akdis M. Blesken T. Bettens F. et al.Successful immunotherapy with T-cell epitope peptides of bee venom phospholipase A2 induces specific T-cell anergy in patients allergic to bee venom.J Allergy Clin Immunol. 1998; 101: 747-754Abstract Full Text Full Text PDF PubMed Scopus (383) Google Scholar IgG4 antibodies do not activate complement and have little or no inflammatory activity. IgG antibodies have therefore been proposed as blocking antibodies by competing with IgE for allergen binding to mast cells, basophils, and other IgE receptor–expressing cells. For example, IgG4 induced by means of immunotherapy blocks allergen-induced IgE-dependent histamine release by basophils17Garcia B.E. Sanz M.L. Gato J.J. Fernandez J. Oehling A. IgG4 blocking effect on the release of antigen-specific histamine.J Investig Allergol Clin Immunol. 1993; 3: 26-33PubMed Google Scholar and suppresses allergen-specific T-cell responses in vitro by inhibiting binding of IgE-allergen complexes to antigen-presenting cells.18van Neerven R.J. Wikborg T. Lund G. Jacobsen B. Brinch-Nielsen A. Arnved J. et al.Blocking antibodies induced by specific allergy vaccination prevent the activation of CD4+ T cells by inhibiting serum-IgE-facilitated allergen presentation.J Immunol. 1999; 163: 2944-2952PubMed Google Scholar, 19Wachholz P.A. Soni N.K. Till S.J. Durham S.R. Inhibition of allergen-IgE binding to B cells by IgG antibodies after grass pollen immunotherapy.J Allergy Clin Immunol. 2003; 112: 915-922Abstract Full Text Full Text PDF PubMed Scopus (254) Google Scholar Another potential anti-inflammatory effect of IgG could be mediated through the coaggregation of inhibitory FcγRIIb receptors with high-affinity FcγRI IgE receptors.20Daeron M. Malbec O. Latour S. Arock M. Fridman W.H. Regulation of high-affinity IgE receptor-mediated mast cell activation by murine low-affinity IgG receptors.J Clin Invest. 1995; 95: 577-585Crossref PubMed Scopus (305) Google Scholar FcγRIIb receptors possess an immunoreceptor tyrosine-based inhibition motif (ITIM) that can be phosphorylated by FcεRI-associated kinases. Phosphorylated ITIM in turn binds intracellular phosphatases that mediate reciprocal inhibition of FcεRI signaling.21Malbec O. Fong D.C. Turner M. Tybulewicz V.L. Cambier J.C. Fridman W.H. et al.Fc epsilon receptor I-associated lyn-dependent phosphorylation of Fc gamma receptor IIB during negative regulation of mast cell activation.J Immunol. 1998; 160: 1647-1658PubMed Google Scholar Thus FcεRI can cause ITIM-dependent autoinhibition of its own signaling. Potentiation of FcγRIIb-FcεRI interaction by a chimeric Fcγ-Fcεconstruct resulted in inhibition of allergen-IgE–dependent histamine release by human basophils.22Zhu D. Kepley C.L. Zhang M. Zhang K. Saxon A. A novel human immunoglobulin Fc gamma Fc epsilon bifunctional fusion protein inhibits Fc epsilon RI-mediated degranulation.Nat Med. 2002; 8: 518-521Crossref PubMed Scopus (179) Google Scholar Whether immunotherapy results in formation of multivalent IgE-IgG allergen complexes that mediate a similar effect is unknown. The particular properties of allergen-specific IgA antibodies induced by means of immunotherapy have also yet to be determined. A traditional objection to the blocking antibody model is the weak correlation between IgG concentrations and the clinical response to treatment.23Ewan P.W. Deighton J. Wilson A.B. Lachmann P.J. Venom-specific IgG antibodies in bee and wasp allergy: lack of correlation with protection from stings.Clin Exp Allergy. 1993; 23: 647-660Crossref PubMed Scopus (87) Google Scholar, 24Djurup R. Malling H.J. High IgG4 antibody level is associated with failure of immunotherapy with inhalant allergens.Clin Allergy. 1987; 17: 459-468Crossref PubMed Scopus (105) Google Scholar For example, immunotherapy in rush protocols is effective long before any changes in antibody synthesis are detected. However, this interpretation might be overly simplistic: Michils et al25Michils A. Mairesse M. Ledent C. Gossart B. Baldassarre S. Duchateau J. Modified antigenic reactivity of anti-phospholipase A2 IgG antibodies in patients allergic to bee venom: conversion with immunotherapy and relation to subclass expression.J Allergy Clin Immunol. 1998; 102: 118-126Abstract Full Text Full Text PDF PubMed Scopus (21) Google Scholar observed the usual increase in IgG titers to venom immunotherapy but showed that this was preceded by a change in the fine specificity of IgG antibodies. Remarkably, changes in allergen-IgG binding were apparent within the first few hours of ultrarush immunotherapy26Michils A. Baldassarre S. Ledent C. Mairesse M. Gossart B. Duchateau J. Early effect of ultrarush venom immunotherapy on the IgG antibody response.Allergy. 2000; 55: 455-462Crossref PubMed Scopus (25) Google Scholar and were sustained when patients were followed up at 6 months.25Michils A. Mairesse M. Ledent C. Gossart B. Baldassarre S. Duchateau J. Modified antigenic reactivity of anti-phospholipase A2 IgG antibodies in patients allergic to bee venom: conversion with immunotherapy and relation to subclass expression.J Allergy Clin Immunol. 1998; 102: 118-126Abstract Full Text Full Text PDF PubMed Scopus (21) Google Scholar Similarly, in murine models of immunotherapy, high concentrations of allergen have been shown to alter antibody affinity and specificity, as well as quantity.27Kolbe L. Heusser C.H. Kolsch E. Isotype-associated recognition of allergen epitopes and its modulation by antigen dose.Immunology. 1995; 84: 285-289PubMed Google Scholar Immunotherapy could act to change both the character and overall amounts of allergen-specific IgG. These observations stress the importance of studying the activity of allergen-specific IgG, as a blocking antibody or otherwise, as opposed to measuring crude levels in sera. The central role of T cells in directing allergic responses has generated a number of hypotheses regarding their role in immunotherapy. Investigations have addressed mechanisms such as immune deviation away from a TH2 phenotype toward a more nonpathogenic TH1 phenotype, inhibition of antigen presentation to allergen-specific T cells, and, more recently, suppression of responses by T cells with regulatory activity. Most studies addressing in vitro T-cell responses to allergen have examined cells isolated from peripheral blood. Early studies of patients treated with venom or inhalant allergen immunotherapy reported a reduction in proliferative responses to allergen,15Jutel M. Akdis M. Budak F. Aebischer-Casaulta C. Wrzyszcz M. Blaser K. et al.IL-10 and TGF-beta cooperate in the regulatory T cell response to mucosal allergens in normal immunity and specific immunotherapy.Eur J Immunol. 2003; 33: 1205-1214Crossref PubMed Scopus (794) Google Scholar, 28Jutel M. Pichler W.J. Skrbic D. Urwyler A. Dahinden C. Muller U.R. Bee venom immunotherapy results in decrease of IL-4 and IL-5 and increase of IFN-gamma secretion in specific allergen-stimulated T cell cultures.J Immunol. 1995; 154: 4187-4194PubMed Google Scholar, 29Akdis C.A. Akdis M. Blesken T. Wymann D. Alkan S.S. Muller U. et al.Epitope-specific T cell tolerance to phospholipase A2 in bee venom immunotherapy and recovery by IL-2 and IL-15 in vitro.J Clin Invest. 1996; 98: 1676-1683Crossref PubMed Scopus (284) Google Scholar, 30Ebner C. Siemann U. Bohle B. Willheim M. Wiedermann U. Schenk S. et al.Immunological changes during specific immunotherapy of grass pollen allergy: reduced lymphoproliferative responses to allergen and shift from TH2 to TH1 in T-cell clones specific for Phl p 1, a major grass pollen allergen.Clin Exp Allergy. 1997; 27: 1007-1015Crossref PubMed Scopus (315) Google Scholar, 31Eusebius N.P. Papalia L. Suphioglu C. McLellan S.C. Varney M. Rolland J.M. et al.Oligoclonal analysis of the atopic T cell response to the group 1 allergen of Cynodon dactylon (Bermuda grass) pollen: pre- and post-allergen-specific immunotherapy.Int Arch Allergy Immunol. 2002; 127: 234-244Crossref PubMed Scopus (35) Google Scholar with an overall shift away from a TH2 to a TH1 response.28Jutel M. Pichler W.J. Skrbic D. Urwyler A. Dahinden C. Muller U.R. Bee venom immunotherapy results in decrease of IL-4 and IL-5 and increase of IFN-gamma secretion in specific allergen-stimulated T cell cultures.J Immunol. 1995; 154: 4187-4194PubMed Google Scholar, 30Ebner C. Siemann U. Bohle B. Willheim M. Wiedermann U. Schenk S. et al.Immunological changes during specific immunotherapy of grass pollen allergy: reduced lymphoproliferative responses to allergen and shift from TH2 to TH1 in T-cell clones specific for Phl p 1, a major grass pollen allergen.Clin Exp Allergy. 1997; 27: 1007-1015Crossref PubMed Scopus (315) Google Scholar, 32Secrist H. Chelen C.J. Wen Y. Marshall J.D. Umetsu D.T. Allergen immunotherapy decreases interleukin 4 production in CD4+ T cells from allergic individuals.J Exp Med. 1993; 178: 2123-2130Crossref PubMed Scopus (551) Google Scholar However, other studies, including 3 of our own based on different cohorts of patients with pollen immunotherapy, have not reproduced these findings.33Till S. Walker S. Dickason R. Huston D. O'Brien F. Lamb J. et al.IL-5 production by allergen-stimulated T cells following grass pollen immunotherapy for seasonal allergic rhinitis.Clin Exp Immunol. 1997; 110: 114-121Crossref PubMed Scopus (35) Google Scholar, 34Wachholz P.A. Nouri-Aria K.T. Wilson D.R. Walker S.M. Verhoef A. Till S.J. et al.Grass pollen immunotherapy for hayfever is associated with increases in local nasal but not peripheral Th1:Th2 cytokine ratios.Immunology. 2002; 105: 56-62Crossref PubMed Scopus (116) Google Scholar, 35Francis J.N. Till S.J. Durham S.R. Induction of IL-10+CD4+CD25+ T cells by grass pollen immunotherapy.J Allergy Clin Immunol. 2003; 111: 1255-1261Abstract Full Text Full Text PDF PubMed Scopus (459) Google Scholar A possible explanation is that inhibition of peripheral T-cell proliferation and TH2 cytokine production is not the fundamental event in immunotherapy. Cultures of PBMCs might provide only a crude reflection of immune interactions and responses in lymphoid tissues, mucosal tissues, or both. In contrast, the detection after immunotherapy of peripheral blood T cells that produce IL-10 in response to allergen stimulation has emerged as a highly consistent finding. Bellinghausen et al36Bellinghausen I. Metz G. Enk A.H. Christmann S. Knop J. Saloga J. Insect venom immunotherapy induces interleukin-10 production and a Th2-to-Th1 shift, and changes surface marker expression in venom-allergic subjects.Eur J Immunol. 1997; 27: 1131-1139Crossref PubMed Scopus (255) Google Scholar were the first to describe IL-10 production after venom immunotherapy. Akdis et al37Akdis C.A. Blesken T. Akdis M. Wuthrich B. Blaser K. Role of interleukin 10 in specific immunotherapy.J Clin Invest. 1998; 102: 98-106Crossref PubMed Scopus (880) Google Scholar similarly described an increase in IL-10 production in response to venom immunotherapy, and this was superimposed on a global suppression of T-cell cytokine and proliferative responses to stimulation with venom allergen in vitro. The same investigators observed a similar IL-10 response to venom allergen in vitro in beekeepers who became naturally tolerized to venom after repetitive stings. An important question concerns whether IL-10 plays a role in the induction of clinical tolerance observed with immunotherapy to inhalant allergens, and 2 recent studies have specifically addressed this issue. Jutel et al15Jutel M. Akdis M. Budak F. Aebischer-Casaulta C. Wrzyszcz M. Blaser K. et al.IL-10 and TGF-beta cooperate in the regulatory T cell response to mucosal allergens in normal immunity and specific immunotherapy.Eur J Immunol. 2003; 33: 1205-1214Crossref PubMed Scopus (794) Google Scholar investigated immunotherapy with house dust mite. Allergen-induced IL-10 and TGF-β production by T cells occurred in parallel to a global suppression of TH2 proliferative responses and cytokine production. IL-10 and TGF-β were detectable at 7 days into a semirush immunotherapy protocol, and this activity copurified with CD4+CD25+ T cells. Similar IL-10 responses were observed in healthy nonatopic subjects exposed to allergen. The implication was that immunotherapy restored a tolerant T-cell response in atopic subjects similar to that observed in healthy individuals. The second study also identified CD4+CD25+ T cells as a source of IL-10 after grass pollen immunotherapy, whereas no changes in grass pollen–induced proliferation or TH2 cytokine production were observed.35Francis J.N. Till S.J. Durham S.R. Induction of IL-10+CD4+CD25+ T cells by grass pollen immunotherapy.J Allergy Clin Immunol. 2003; 111: 1255-1261Abstract Full Text Full Text PDF PubMed Scopus (459) Google Scholar Although allergen-dependent IL-10 production is detectable in peripheral blood T cells, it is likely that in mucosal diseases, such as allergic rhinitis, the clinically important effects of IL-10 occur within the mucosa or possibly within the draining lymphoid tissue. Local factors to be considered include the ratio of IL-10–producing cells to other cells, cell-cell contact, the effect of microenvironmental cytokines, the presence of mature antigen-presenting cells, and cytokine trapping by extracellular matrix proteins. An important question is how allergen-specific T cells are recruited to allergic tissue sites. A possible mechanism underlying immunotherapy might be altered chemokine receptor expression by peripheral blood T cells. Chemokines are small chemoattractant molecules that act through specific receptors on leukocytes. Different subsets of immune cells, such as TH2 cells, might be subject to selective recruitment as a result of restricted expression of certain chemokine receptors. Recent studies have identified the presence of functional CXCR1 on CD4+ T cells from allergic patients but not on those from control subjects. CXCR1 is a receptor for IL-8 and has traditionally been associated with high surface expression on neutrophils. However, as a result of screening atopic allergic subjects and nonatopic control subjects for T-cell surface expression for a whole panel of known chemokine receptors, CXCR1 emerged consistently as a prominent T-cell surface receptor associated with allergic inflammation, including allergen-induced late asthmatic responses.38Francis J.N. Jacobson M.R. Lloyd C.M. Sabroe I. Durham S.R. Till S.J. CXCR1+CD4+ T cells in human allergic disease.J Immunol. 2004; 172: 268-273Crossref PubMed Scopus (18) Google Scholar When compared with current symptomatic patients with allergic rhinitis, CXCR1 expression on circulating CD4+ T cells from patients receiving grass pollen immunotherapy was low or absent.38Francis J.N. Jacobson M.R. Lloyd C.M. Sabroe I. Durham S.R. Till S.J. CXCR1+CD4+ T cells in human allergic disease.J Immunol. 2004; 172: 268-273Crossref PubMed Scopus (18) Google Scholar The effect of immunotherapy on other T-cell chemokine receptors remains to be determined. T-cell responses after grass pollen immunotherapy have been examined in nasal mucosal and skin tissue after grass pollen immunotherapy. Nasal and cutaneous biopsy specimens were collected from patients and allergic control subjects during randomized controlled trials of immunotherapy, either after allergen challenge or during natural seasonal exposure. Cytokine production was examined in vivo by use of in situ hybridization with specific riboprobes, identifying specific cytokine mRNAs. Although treatment appeared to be associated with reduced accumulation of T cells in skin and nose after allergen challenge, there was no attenuation of T-cell numbers in the nasal mucosa during natural seasonal exposure, suggesting that factors other than a reduction in total T cells must account for clinical efficacy. The first study to describe a shift of allergen-induced T-cell responses in favor of TH1 cytokine synthesis was published by Varney et al in 1993.39Varney V.A. Hamid Q.A. Gaga M. Ying S. Jacobson M. Frew A.J. et al.Influence of grass pollen immunotherapy on cellular infiltration and cytokine mRNA expression during allergen-induced late-phase cutaneous responses.J Clin Invest. 1993; 92: 644-651Crossref PubMed Scopus (470) Google Scholar After 1 year of a controlled trial of grass pollen immunotherapy, intradermal challenge with grass pollen extract was associated with a reduction in the cutaneous late-phase response. When this site underwent biopsy at 24 hours, contrary to expectations, a significant reduction in numbers of IL-4 or IL-5 mRNA–expressing cells was not observed. On the other hand, modest but significant increases in IFN-γ and IL-2 mRNA–expressing cells were observed, consistent with local immune deviation in favor of a TH1 response. Subsequently, skin biopsy specimens collected after 4 years of immunotherapy were examined for expression of mRNA encoding IL-12, a potent regulator of TH1 responses.40Varga E.M. Wachholz P. Nouri-Aria K.T. Verhoef A. Corrigan C.J. Till S.J. et al.T cells from human allergen-induced late asthmatic responses express IL-12 receptor beta 2 subunit mRNA and respond to IL-12 in vitro.J Immunol. 2000; 165: 2877-2885PubMed Google Scholar IL-12 mRNA expression did indeed increase after immunotherapy and correlated positively with IFN-γ mRNA expression and inversely with IL-4 expression.41Hamid Q.A. Schotman E. Jacobson M.R. Walker S.M. Durham S.R. Increases in IL-12 messenger RNA+ cells accompany inhibition of allergen-induced late skin responses after successful grass pollen immunotherapy.J Allergy Clin Immunol. 1997; 99: 254-260Abstract Full Text Full Text PDF PubMed Scopus (217) Google Scholar It seems likely that in allergy to inhaled allergens, it is studies of the immunologic changes within the respiratory mucosa (ie, the site of disease) that are of greatest relevance. Nasal