Transferred T cells preferentially adhere in the BALT of CD26-deficient recipient lungs during asthma

CD26 and Asthma: a Comprehensive Review

Asthma is a heterogeneous and chronic inflammatory family of disorders of the airways with increasing prevalence that results in recurrent and reversible bronchial obstruction and expiratory airflow limitation. These diseases arise from the interaction between environmental and genetic factors, which collaborate to cause increased susceptibility and severity. Many asthma susceptibility genes are linked to the immune system or encode enzymes like metalloproteases (e.g., ADAM-33) or serine proteases. The S9 family of serine proteases (prolyl oligopeptidases) is capable to process peptide bonds adjacent to proline, a kind of cleavage-resistant peptide bonds present in many growth factors, chemokines or cytokines that are important for asthma. Curiously, two serine proteases within the S9 family encoded by genes located on chromosome 2 appear to have a role in asthma: CD26/dipeptidyl peptidase 4 (DPP4) and DPP10. The aim of this review is to summarize the current knowledge about CD26 and to provide a structured overview of the numerous functions and implications that this versatile enzyme could have in this disease, especially after the detection of some secondary effects (e.g., viral nasopharyngitis) in type II diabetes mellitus patients (a subset with a certain risk of developing obesity-related asthma) upon CD26 inhibitory therapy.

Cut to the chase: a review of CD26/dipeptidyl peptidase-4's (DPP4) entanglement in the immune system

CD26/DPP4 (dipeptidyl peptidase 4/DP4/DPPIV) is a surface T cell activation antigen and has been shown to have DPP4 enzymatic activity, cleaving-off amino-terminal dipeptides with either L-proline or L-alanine at the penultimate position. It plays a major role in glucose metabolism by N-terminal truncation and inactivation of the incretins glucagon-like peptide-1 (GLP) and gastric inhibitory protein (GIP). In 2006, DPP4 inhibitors have been introduced to clinics and have been demonstrated to efficiently enhance the endogenous insulin secretion via prolongation of the half-life of GLP-1 and GIP in patients. However, a large number of studies demonstrate clearly that CD26/DPP4 also plays an integral role in the immune system, particularly in T cell activation. Therefore, inhibition of DPP4 might represent a double-edged sword. Apart from the metabolic benefit, the associated immunological effects of long term DPP4 inhibition on regulatory processes such as T cell homeostasis, maturation and activation are not understood fully at this stage. The current data point to an important role for CD26/DPP4 in maintaining lymphocyte composition and function, T cell activation and co-stimulation, memory T cell generation and thymic emigration patterns during immune-senescence. In rodents, critical immune changes occur at baseline levels as well as after in-vitro and in-vivo challenge. In patients receiving DPP4 inhibitors, evidence of immunological side effects also became apparent. The scope of this review is to recapitulate the role of CD26/DPP4 in the immune system regarding its pharmacological inhibition and T cell-dependent immune regulation.

Dipeptidyl peptidase IV (DPP4) deficiency increases Th1-driven allergic contact dermatitis

BACKGROUND

CD26 or dipeptidyl peptidase IV (DPP4) is known to be involved in several immunological processes and has recently been reported to play a crucial role in the allergic responses of the lungs.

OBJECTIVES

To explore the impact of DPP4 on the allergic response of the skin.

METHODS

Skin biopsies from patients suffering from atopic dermatitis (AD) and healthy controls were investigated for the expression of CD26/DPP4. Furthermore, the functional impact of CD26 was investigated in two models of contact hypersensitivity using CD26/DPP4-deficient and wild-type rats. Dinitrochlorobenzene (DNCB) was used to induce a T helper type 1 (Th1)-dominated inflammation and toluene-2,3-diisocyanate for a Th2-pronounced inflammation. The inflammatory responses were determined by histological quantification, flow cytometry [fluorescence-activated cell sorting (FACS)], and an enzyme-linked immunosorbant assay (ELISA).

RESULTS

CD26/DPP4-expression was up-regulated in the lesional skin biopsies of patients compared with healthy controls as well as in both models of contact hypersensitivity. However, in the more Th2-driven model, a reduced inflammatory skin response was found in CD26/DPP4-deficient rats, analogous to the effects observed recently in a rat model of asthma. In partial contrast, there was an aggravation of local skin inflammation in CD26/DPP4-deficient rats under conditions of Th1-like skin inflammation.

CONCLUSION AND CLINICAL RELEVANCE

The up-regulation of CD26 in atopic dermatitis represents a new finding, which has also been seen in other inflammatory skin diseases. However, tissue expression of CD26/DPP4 in immunological skin response can either be beneficial or aggravating, depending on a possible Th1/Th2 shift. This might have consequences for humans suffering from diabetes mellitus treated by DPP4 inhibitors, who have eczematous skin diseases as a co-morbidity.

Bronchus-associated lymphoid tissue (BALT) structure and function

Bronchus-associated lymphoid tissue (BALT) is a constitutive mucosal lymphoid tissue adjacent to major airways in some mammalian species, including rats and rabbits, but not humans or mice. A related tissue, inducible BALT (iBALT), is an ectopic lymphoid tissue that is formed upon inflammation or infection in both mice and humans and can be found throughout the lung. Both BALT and iBALT acquire antigens from the airways and initiate local immune responses and maintain memory cells in the lungs. Here, we discuss the development and function of BALT and iBALT in the context of pulmonary immunity to infectious agents, tumors, and allergens as well as autoimmunity and inflammatory diseases of the lung.

T regulatory cells and attenuated bleomycin-induced fibrosis in lungs of CCR7-/- mice

Background

C-C chemokine receptor (CCR)7 is a regulator of dendritic cell and T cell migration, and its role in tissue wound healing has been investigated in various disease models. We have previously demonstrated that CCR7 and its ligand, chemokine (C-C motif) ligand (CCL)21, modulates wound repair in pulmonary fibrosis (PF) but the mechanism of this is unknown. The objective of this study was to investigate whether the absence of CCR7 protects against bleomycin (BLM)-induced PF. CCR7^-/- mice failed to mount a fibrotic pulmonary response as assessed by histologic collagen staining and quantification by hydroxyproline. We hypothesized that the prominent characteristics of CCR7^-/- mice, including elevated levels of cytokine and chemokine mediators and the presence of bronchus-associated lymphoid tissue (BALT) might be relevant to the protective phenotype.

Results

Pulmonary fibrosis was induced in CCR7^+/+ and CCR7^-/- mice via a single intratracheal injection of BLM. We found that the lung cytokine/chemokine milieu associated with the absence of CCR7 correlated with an increase in BALT, and might be attributable to regulatory T cell (Treg) homeostasis and trafficking within the lungs and lymph nodes. In response to BLM challenge, CCR7^-/- mice exhibited an early, steady increase in lung CD4⁺ T cells and increased CD4⁺ CD25⁺ FoxP3⁺ Tregs in the lungs 21 days after challenge. These findings are consistent with increased lung expression of interleukin-2 and indoleamine 2,3-dioxygenase in CCR7^-/- mice, which promote Treg expansion.

Conclusions

Our study demonstrates that the protective phenotype associated with BLM-treated CCR7^-/- mice correlates with the presence of BALT and the anchoring of Tregs in the lungs of CCR7^-/- mice. These data provide novel evidence to support the further investigation of CCR7-mediated Treg trafficking in the modulation of BLM-induced PF.