Dendritic cells produce CXCL13 and participate in the development of murine small intestine lymphoid tissues

Defining the anatomical localisation of subsets of the murine mononuclear phagocyte system using integrin alpha X (Itgax, CD11c) and colony stimulating factor 1 receptor (Csf1r, CD115) expression fails to discriminate dendritic cells from macrophages

The murine mononuclear phagocyte (MNP) system comprises a diverse population of cells, including monocytes, dendritic cells (DC) and macrophages. Derived from the myeloid haematopoietic lineage, this group of cells express a variety of well characterized surface markers. Expression of the integrin alpha X (Itgax, CD11c) is commonly used to identify classical DC, and similarly expression of colony stimulating factor 1 receptor (Csf1r, CD115) to identify macrophages. We have characterized the expression of these markers using a variety of transgenic mouse models. We confirmed previous observations of Itgax expression in anatomically defined subsets of MNPs in secondary lymphoid organs, including all MNPs identified within the germinal centres. The majority of MNPs in the intestinal lamina propria and lung express Itgax. All mucosal Itgax expressing cells also express Csf1r suggesting Csf1-dependent haematopoietic derivation. This double-positive population included germinal centre MNPs. These data reveal that Itgax expression alone does not specifically define classical DC. These results suggest more cautious interpretation of Itgax-dependent experimentation and direct equation with uniquely DC-mediated activities, particularly in the functioning of non-lymphoid MNPs within the intestinal lamina propria.

GALT: organization and dynamics leading to IgA synthesis

Since its discovery more than four decades ago, immunoglobulin (Ig) A has been the subject of continuous and intensive studies. The major concepts derived were that the precursors of IgA plasma cells are generated in follicular organized structures with the help of T cells and the secreted IgAs provide protection against mucosal pathogens. However, only recently we began to appreciate that IgAs play key roles in regulation of bacterial communities in the intestine and that the repertoire of gut microbiota is closely linked to the proper functioning of the immune system. In this review, we highlight the complex and dynamic mutualistic relationships between bacteria and immune cells and discuss the sites and pathways leading to IgA synthesis in gut-associated lymphoid tissues (GALT).

Aging impacts isolated lymphoid follicle development and function

BACKGROUND

Immunosenescence is the age-related decline and dysfunction of protective immunity leading to a marked increase in the risk of infections, autoimmune disease, and cancer. The majority of studies have focused on immunosenescence in the systemic immune system; information concerning the effect of aging on intestinal immunity is limited. Isolated lymphoid follicles (ILFs) are newly appreciated dynamic intestinal lymphoid structures that arise from nascent lymphoid tissues, or cryptopatches (CP), in response to local inflammatory stimuli. ILFs promote "homeostatic" responses including the production of antigen-specific IgA, thus playing a key role in mucosal immune protection. ILF dysfunction with aging could contribute to immunosenescence of the mucosal system, and accordingly we examined phenotypic and functional aspects of ILFs from young (2 month old) and aged (2 year old) mice.

RESULTS

We observed that aged mice have increased numbers of ILFs and increased numbers of structures corresponding to an early stage of CPs transforming into ILFs. The cellular composition of ILFs in aged mice is altered with a smaller B-lymphocyte population and an increased T-lymphocyte population. The ILF T-lymphocyte population is notable by the presence of CD4+ CD8αα+ T-lymphocytes, which are absent from the systemic compartment. The smaller B-lymphocyte population in ILFs from aged mice is directly correlated with decreased mRNA and protein expression of CCL20 and CXCL13, two chemokines that play crucial roles in recruiting B-lymphocytes into ILFs. Aged mice had elevated levels of serum and fecal immunoglobulins and despite the decreased B-lymphocyte population, ILFs from aged mice displayed increased IgA production. The immunoglobulin repertoire was skewed in aged mice, and ILFs demonstrated a repertoire usage similar to that of the systemic pool in both young and aged mice.

CONCLUSIONS

Here we observed that ILF development, cellular composition, and immunoglobulin production are altered with aging suggesting that ILF dysfunction contributes to mucosal immunosenescence.

Oestrogen-mediated protection of experimental autoimmune encephalomyelitis in the absence of Foxp3+ regulatory T cells implicates compensatory pathways including regulatory B cells

Oestrogen (17β-oestradiol, E₂) is a highly effective treatment for experimental autoimmune encephalomyelitis (EAE) that may potentiate Foxp3+ regulatory T (Treg) cells, which in turn limit the expansion of encephalitogenic T-cell specificities. To determine if Treg cells constitute the major non-redundant protective pathway for E₂, we evaluated E₂ protection of EAE after targeted deletion of Foxp3 expression in Foxp3-DTR mice. Unexpectedly, E₂-treated Foxp3-deficient mice were completely protected against clinical and histological myelin oligodendrocyte glycoprotein (MOG)-35-55 peptide-induced EAE before succumbing to diphtheria toxin-induced mortality. This finding indicated the presence of alternative E₂-dependent EAE-protective pathways that could compensate for the lack of Treg cells. Further investigation revealed that E₂ treatment inhibited proliferation and expression of CCL2 and CXCL2, but enhanced secretion of interleukin-10 (IL-10) and IL-13 by MOG-35-55-specific spleen cells. These changes occurred concomitantly with increased expression of several chemokines and receptors, including CXCL13 and CXCR5, and the negative co-activation molecules, PD-L1 and B7.2, by B cells and dendritic cells. Furthermore, E₂ treatment resulted in higher percentages of spleen and lymph node T cells expressing IL-17, interferon-γ and tumour necrosis factor-α, but with lower expression of CCR6, suggesting sequestration of MOG-35-55 peptide-specific T cells in peripheral immune organs. Taken together, these data suggest that E₂-induced mechanisms that provide protection against EAE in the absence of Foxp3+ Treg cells include induction of regulatory B cells and peripheral sequestration of encephalitogenic T cells.

New insights into the development of lymphoid tissues

Secondary lymphoid organs are important locations for the initiation of adaptive immune responses. They develop before birth, and their formation requires interaction between lymphotoxin-α₁ß₂-expressing lymphoid-tissue inducer cells and lymphotoxin-ß receptor-expressing stromal organizer cells. Here, we discuss new insights into the earliest phases of peripheral lymph node and Peyer's patch formation that occur before lymphotoxin-ß receptor signalling and suggest a role for the developing nervous system. In addition, we discuss the differing requirements for the postnatal formation of mucosa-associated lymphoid tissues and tertiary lymphoid structures that develop at sites of chronic inflammation.