Evidence that a significant number of naive T cells enter non-lymphoid organs as part of a normal migratory pathway

Evidence for a functional thymic stromal lymphopoietin signaling axis in fibrotic lung disease

Thymic stromal lymphopoietin (TSLP) recently has emerged as a key cytokine in the development of type 2 immune responses. Although traditionally associated with allergic inflammation, type 2 responses are also recognized to contribute to the pathogenesis of tissue fibrosis. However, the role of TSLP in the development of non-allergen-driven diseases, characterized by profibrotic type 2 immune phenotypes and excessive fibroblast activation, remains underexplored. Fibroblasts represent the key effector cells responsible for extracellular matrix production but additionally play important immunoregulatory roles, including choreographing immune cell recruitment through chemokine regulation. The aim of this study was to examine whether TSLP may be involved in the pathogenesis of a proto-typical fibrotic disease, idiopathic pulmonary fibrosis (IPF). We combined the immunohistochemical analysis of human IPF biopsy material with signaling studies by using cultured primary human lung fibroblasts and report for the first time, to our knowledge, that TSLP and its receptor (TSLPR) are highly upregulated in IPF. We further show that lung fibroblasts represent both a novel cellular source and target of TSLP and that TSLP induces fibroblast CCL2 release (via STAT3) and subsequent monocyte chemotaxis. These studies extend our understanding of TSLP as a master regulator of type 2 immune responses beyond that of allergic inflammatory conditions and suggest a novel role for TSLP in the context of chronic fibrotic lung disease.

CD4(+) T-cell subsets in intestinal inflammation

Intestinal CD4⁺ T cells are essential mediators of immune homeostasis and inflammation. Multiple subsets of CD4⁺ T cells have been described in the intestine, which represents an important site for the generation and regulation of cells involved in immune responses both within and outside of the gastrointestinal tract. Recent advances have furthered our understanding of the biology of such cells in the intestine. Appreciation of the functional roles for effector and regulatory populations in health and disease has revealed potential translational targets for the treatment of intestinal diseases, including inflammatory bowel disease. Furthermore, the role of dietary and microbiota-derived factors in shaping the intestinal CD4⁺ T-cell compartment is becoming increasingly understood. Here, we review recent advances in understanding the multifaceted roles of CD4⁺ T cells in intestinal immunity.

Enhanced and aberrant T cell trafficking following total body irradiation: a gateway to graft-versus-host disease?

Pre-transplant conditioning regimens play a major role in triggering graft-versus-host disease (GVHD). This study investigated the effect of irradiation on donor T cell trafficking to lymphoid and non-lymphoid tissues by comparing the migration of carboxy-fluorescein diacetate succinimidyl ester-labelled, naïve donor T lymphocytes in vivo in irradiated and non-irradiated syngeneic mice recipients. Recruitment of adoptively transferred naïve T cells to secondary lymphoid organs was increased in irradiated mice and naïve T cells also aberrantly localized to non-lymphoid tissues. Irradiation also induced aberrant effector memory T cell migration into lymph nodes and their localization to homing-privileged non-lymphoid sites, such as the gut. The presence of a minor histocompatibility mismatch further enhanced the aberrant accumulation of T cells in both lymphoid and non-lymphoid tissue, whilst their migratory pattern was not modified as compared to fully matched irradiated recipients. These effects correlated with decreased permeability of, and the secretion of chemotactic factors by the endothelium. Our findings are consistent with the possibility that excessive, dysregulated extravasation of T cells induced by irradiation promotes the development of GVHD.

Extranodal induction of therapeutic immunity in the tumor microenvironment after intratumoral delivery of Tbet gene-modified dendritic cells

Murine dendritic cells (DC) transduced to express the Type-1 transactivator T-bet (i.e. mDC.Tbet) and delivered intratumorally as a therapy are superior to control wild-type DC in slowing the growth of established subcutaneous MCA205 sarcomas in vivo. Optimal antitumor efficacy of mDC.Tbet-based gene therapy was dependent on host natural killer (NK) cells and CD8(+) T cells, and required mDC.Tbet expression of major histocompatibility complex class I molecules, but was independent of the capacity of the injected mDC.Tbet to produce proinflammatory cytokines (interleukin-12 family members or interferon-γ) or to migrate to tumor-draining lymph nodes based on CCR7 ligand chemokine recruitment. Conditional (CD11c-DTR) or genetic (BATF3(-/-)) deficiency in host antigen-crosspresenting DC did not diminish the therapeutic action of intratumorally delivered wild-type mDC.Tbet. Interestingly, we observed that intratumoral delivery of mDC.Tbet (versus control mDC.Null) promoted the acute infiltration of NK cells and naive CD45RB(+) T cells into the tumor microenvironment (TME) in association with elevated expression of NK- and T-cell-recruiting chemokines by mDC.Tbet. When taken together, our data support a paradigm for extranodal (cross)priming of therapeutic Type-1 immunity in the TME after intratumoral delivery of mDC.Tbet-based gene therapy.

PLGA nanoparticle entrapped killed porcine reproductive and respiratory syndrome virus vaccine helps in viral clearance in pigs

Porcine reproductive and respiratory syndrome (PRRS) is a chronic viral disease of pigs, has been posing a huge economic concern to pig industry worldwide. In this study, we developed biodegradable PLGA [poly(d,l-lactide-co-glycolide)] nanoparticle-entrapped killed PRRSV vaccine (Nano-KAg), and administered intranasally to pigs once and evaluated the immune correlates. In Nano-KAg vaccinated homologous virus challenged pigs, complete clearance of viremia was observed in 2 weeks, associated with a significant increase in virus neutralizing titers only in the lungs, compared to both unvaccinated and killed vaccine vaccinated pigs. The lung homogenate and sera of Nano-KAg vaccinated pigs had higher levels of IFN-γ and lower levels of TGF-β than control groups. Restimulation of mononuclear cells isolated from the lungs, blood, BAL, and TBLN of Nano-KAg vaccinated pigs’ secreted significantly increased levels of Th1 cytokines, IFN-γ and IL-12. In addition, higher frequencies of CD3⁺CD8⁺, CD4⁺CD8⁺, and γδ T cells, and reduced frequency of Foxp3⁺ T-regulatory cells were observed in Nano-KAg vaccinated pigs. Thus, intranasal delivery of Nano-KAg vaccine may be a suitable strategy to elicit anti-PRRSV immune response required to better clear viremia in pigs.

Efficiently stimulated adult microglia cross-prime naive CD8+ T cells injected in the brain

Microglia are the major myeloid-immune cells of the brain parenchyma. In a steady state, microglia monitor their environment for pathogens or damaged cells. In response to neural injury or inflammation, microglia become competent APCs able to prime CD4(+) and CD8(+) T lymphocytes. We previously demonstrated that neonatal and adult microglia cross-present exogenous soluble Ags in vitro. However, whether microglia are able to cross-present Ag to naive CD8(+) T cells in vivo, within the brain microenvironment, remains undetermined. Here, we have designed an original protocol in order to exclude the involvement in cross-presentation activity of peripheral migrating APCs and of CNS-associated APCs. In C57Bl/6 mice, in which the body but not the head has been properly irradiated, we analyzed the ability of resident microglia to stimulate intracerebrally injected CD8(+) T cells in vivo. This study demonstrates for the first time that adult microglia cross-present Ag to naive CD8(+) T cells in vivo and that full microglia activation is required to overcome the inhibitory constrains of the brain and to render microglia able to cross-prime naive CD8(+) T cells injected in the brain. These observations offer new insights in brain-tumor immunotherapy based on the induction of cytotoxic antitumoral T cells.

Lung-resident tissue macrophages generate Foxp3+ regulatory T cells and promote airway tolerance

Airway tolerance is the usual outcome of inhalation of harmless antigens. Although T cell deletion and anergy are likely components of tolerogenic mechanisms in the lung, increasing evidence indicates that antigen-specific regulatory T cells (inducible Treg cells [iTreg cells]) that express Foxp3 are also critical. Several lung antigen-presenting cells have been suggested to contribute to tolerance, including alveolar macrophages (MØs), classical dendritic cells (DCs), and plasmacytoid DCs, but whether these possess the attributes required to directly promote the development of Foxp3(+) iTreg cells is unclear. Here, we show that lung-resident tissue MØs coexpress TGF-β and retinal dehydrogenases (RALDH1 and RALDH 2) under steady-state conditions and that their sampling of harmless airborne antigen and presentation to antigen-specific CD4 T cells resulted in the generation of Foxp3(+) Treg cells. Treg cell induction in this model depended on both TGF-β and retinoic acid. Transfer of the antigen-pulsed tissue MØs into the airways correspondingly prevented the development of asthmatic lung inflammation upon subsequent challenge with antigen. Moreover, exposure of lung tissue MØs to allergens suppressed their ability to generate iTreg cells coincident with blocking airway tolerance. Suppression of Treg cell generation required proteases and TLR-mediated signals. Therefore, lung-resident tissue MØs have regulatory functions, and strategies to target these cells might hold promise for prevention or treatment of allergic asthma.

Analysis of naïve lung CD4 T cells provides evidence of functional lung to lymph node migration

The proportion of CD4 T cells with phenotypic and functional properties of naïve cells out of total CD4 T cells is similar in the lung parenchyma and lymph nodes. On treatment with a sphingosine-1-phosphate agonist, the frequency of these cells falls precipitously, but with a delay of ∼14 h compared with blood CD4 T cells; neither anti-CD62L nor pertussis toxin prevents entry of naïve CD4 T cells into the lung. Based on treatment with anti-CD62L and the use of CCR7(-/-) cells, lung naïve CD4 T cells appear to migrate to the mediastinal lymph nodes along a CD62L-independent, CCR7-dependent pathway. Cells that have entered the node in this manner are competent to respond to antigen. Thus, a portion (approximately one-half) of naïve CD4 T cells appears to enter the mediastinal lymph nodes through a blood-to-lung-to-lymph node route.

The role of innate and adaptive immunity in Parkinson's disease

In recent years, inflammation has become implicated as a major pathogenic factor in the onset and progression of Parkinson's disease. Understanding the precise role for inflammation in PD will likely lead to understanding of how sporadic disease arises. In vivo evidence for inflammation in PD includes microglial activation, increased expression of inflammatory genes in the periphery and in the central nervous system (CNS), infiltration of peripheral immune cells into the CNS, and altered composition and phenotype of peripheral immune cells. These findings are recapitulated in various animal models of PD and are reviewed herein. Furthermore, we examine the potential relevance of PD-linked genetic mutations to altered immune function and the extent to which environmental exposures that recapitulate these phenotypes, which may lead to sporadic PD through similar mechanisms. Given the implications of immune system involvement on disease progression, we conclude by reviewing the evidence supporting the potential efficacy of immunomodulatory therapies in PD prevention or treatment. There is a clear need for additional research to clarify the role of immunity and inflammation in this chronic, neurodegenerative disease.

Loss of Neuronal Phenotype and Neurodegeneration: Effects of T Lymphocytes and Brain Interleukin-2

Loss of neuronal phenotype and reversal of neuronal atrophy have been demonstrated in different models of central nervous system (CNS) injury. These processes may be generalizable to different types of brain neurons and circuitry. The idea that some injured neurons may lose their phenotype and/or atrophy with the potential to rejuvenate is a remarkable and potentially promising form of neuronal plasticity that is not well understood. In this paper, we present some of our laboratory's basic neuroimmunology research showing that peripheral T cells entering the CNS, and brain-derived interleukin-2 (IL-2), play significant roles in these intriguing processes. Our findings suggest, for example, that T cell immunosenesence could be involved in related processes of brain aging and contribute to neurodegenerative disease. Neuroimmunological approaches may provide new insights into yet undiscovered factors and brain mechanisms that regulate changes in neuronal integrity associated with aging and disease. Such findings could have important implications for discovering more effective strategies for treating patients with neurotrauma and neurodegenerative diseases (e.g., Alzheimer's disease).

Memory T cell subsets, migration patterns, and tissue residence

Tissues such as the skin and mucosae are frequently exposed to microbial pathogens. Infectious agents must be quickly and efficiently controlled by our immune system, but the low frequency of naive T cells specific for any one pathogen means dependence on primary responses initiated in draining lymph nodes, often allowing time for serious infection to develop. These responses imprint effectors with the capacity to home to infected tissues; this process, combined with inflammatory signals, ensures the effective targeting of primary immunity. Upon vaccination or previous pathogen exposure, increased pathogen-specific T cell numbers together with altered migratory patterns of memory T cells can greatly improve immune efficacy, ensuring infections are prevented or at least remain subclinical. Until recently, memory T cell populations were considered to comprise central memory T cells (TCM), which are restricted to the secondary lymphoid tissues and blood, and effector memory T cells (TEM), which broadly migrate between peripheral tissues, the blood, and the spleen. Here we review evidence for these two memory populations, highlight a relatively new player, the tissue-resident memory T cell (TRM), and emphasize the potential differences between the migratory patterns of CD4(+) and CD8(+) T cells. This new understanding raises important considerations for vaccine design and for the measurement of immune parameters critical to the control of infectious disease, autoimmunity, and cancer.

Cutting edge: intravascular staining redefines lung CD8 T cell responses

Nonlymphoid T cell populations control local infections and contribute to inflammatory diseases, thus driving efforts to understand the regulation of their migration, differentiation, and maintenance. Numerous observations indicate that T cell trafficking and differentiation within the lung are starkly different from what has been described in most nonlymphoid tissues, including intestine and skin. After systemic infection, we found that >95% of memory CD8 T cells isolated from mouse lung via standard methods were actually confined to the pulmonary vasculature, despite perfusion. A respiratory route of challenge increased virus-specific T cell localization within lung tissue, although only transiently. Removing blood-borne cells from analysis by the simple technique of intravascular staining revealed distinct phenotypic signatures and chemokine-dependent trafficking restricted to Ag-experienced T cells. These results precipitate a revised model for pulmonary T cell trafficking and differentiation and a re-evaluation of studies examining the contributions of pulmonary T cells to protection and disease.

Lymph node homing of T cells and dendritic cells via afferent lymphatics

The continuous migration of immune cells is of utmost importance for the induction of both protective immunity as well as immunological tolerance. However, relatively little is known about the molecular cues that regulate the entry of immune cells from peripheral, nonlymphoid tissues into afferent lymph vessels and, in particular, their subsequent transmigration from afferent lymphatics into the parenchyma of draining lymph nodes (LNs). Here, we review the requirements for T cells and dendritic cells (DCs) to enter initial afferent lymph vessels of the skin. We discuss how these cells subsequently gain access to the paracortex of draining lymph nodes; a location that allows for efficient interaction between both cell populations, providing the right environment for the induction of immunity as well as tolerance.

Surfactant protein A integrates activation signal strength to differentially modulate T cell proliferation

Pulmonary surfactant lipoproteins lower the surface tension at the alveolar-airway interface of the lung and participate in host defense. Previous studies reported that surfactant protein A (SP-A) inhibits lymphocyte proliferation. We hypothesized that SP-A-mediated modulation of T cell activation depends upon the strength, duration, and type of lymphocyte activating signals. Modulation of T cell signal strength imparted by different activating agents ex vivo and in vivo in different mouse models and in vitro with human T cells shows a strong correlation between strength of signal (SoS) and functional effects of SP-A interactions. T cell proliferation is enhanced in the presence of SP-A at low SoS imparted by exogenous mitogens, specific Abs, APCs, or in homeostatic proliferation. Proliferation is inhibited at higher SoS imparted by different doses of the same T cell mitogens or indirect stimuli such as LPS. Importantly, reconstitution with exogenous SP-A into the lungs of SP-A(-/-) mice stimulated with a strong signal also resulted in suppression of T cell proliferation while elevating baseline proliferation in unstimulated T cells. These signal strength and SP-A-dependent effects are mediated by changes in intracellular Ca(2+) levels over time, involving extrinsic Ca(2+)-activated channels late during activation. These effects are intrinsic to the global T cell population and are manifested in vivo in naive as well as memory phenotype T cells. Thus, SP-A appears to integrate signal thresholds to control T cell proliferation.

Cutting edge: Tissue-retentive lung memory CD4 T cells mediate optimal protection to respiratory virus infection

We identify in this article a new class of lung tissue-resident memory CD4 T cells that exhibit tissue tropism and retention independent of Ag or inflammation. Tissue-resident memory CD4 T cells in the lung did not circulate or emigrate from the lung in parabiosis experiments, were protected from in vivo Ab labeling, and expressed elevated levels of CD69 and CD11a compared with those of circulating memory populations. Importantly, influenza-specific lung-resident memory CD4 T cells served as in situ protectors to respiratory viral challenge, mediating enhanced viral clearance and survival to lethal influenza infection. By contrast, memory CD4 T cells isolated from spleen recirculated among multiple tissues without retention and failed to mediate protection to influenza infection, despite their ability to expand and migrate to the lung. Our results reveal tissue compartmentalization as a major determining factor for immune-mediated protection in a key mucosal site, important for targeting local protective responses in vaccines and immunotherapies.

In vivo imaging of lymphocytes in the CNS reveals different behaviour of naïve T cells in health and autoimmunity

Background

Two-photon laser scanning microscopy (TPLSM) has become a powerful tool in the visualization of immune cell dynamics and cellular communication within the complex biological networks of the inflamed central nervous system (CNS). Whereas many previous studies mainly focused on the role of effector or effector memory T cells, the role of naïve T cells as possible key players in immune regulation directly in the CNS is still highly debated.

Methods

We applied ex vivo and intravital TPLSM to investigate migratory pathways of naïve T cells in the inflamed and non-inflamed CNS. MACS-sorted naïve CD4+ T cells were either applied on healthy CNS slices or intravenously injected into RAG1 -/- mice, which were affected by experimental autoimmune encephalomyelitis (EAE). We further checked for the generation of second harmonic generation (SHG) signals produced by extracellular matrix (ECM) structures.

Results

By applying TPLSM on living brain slices we could show that the migratory capacity of activated CD4+ T cells is not strongly influenced by antigen specificity and is independent of regulatory or effector T cell phenotype. Naïve T cells, however, cannot find sufficient migratory signals in healthy, non-inflamed CNS parenchyma since they only showed stationary behaviour in this context. This is in contrast to the high motility of naïve CD4+ T cells in lymphoid organs. We observed a highly motile migration pattern for naïve T cells as compared to effector CD4+ T cells in inflamed brain tissue of living EAE-affected mice. Interestingly, in the inflamed CNS we could detect reticular structures by their SHG signal which partially co-localises with naïve CD4+ T cell tracks.

Conclusions

The activation status rather than antigen specificity or regulatory phenotype is the central requirement for CD4+ T cell migration within healthy CNS tissue. However, under inflammatory conditions naïve CD4+ T cells can get access to CNS parenchyma and partially migrate along inflammation-induced extracellular SHG structures, which are similar to those seen in lymphoid organs. These SHG structures apparently provide essential migratory signals for naïve CD4+ T cells within the diseased CNS.

Epithelial transglutaminase 2 is needed for T cell interleukin-17 production and subsequent pulmonary inflammation and fibrosis in bleomycin-treated mice

Inhibition of transglutaminase 2 reduces bleomycin-induced epithelial cell release of interleukin 6 in vitro and pulmonary inflammation and fibrosis in vivo.

Pulmonary fibrosis is a potentially life-threatening disease that may be caused by overt or asymptomatic inflammatory responses. However, the precise mechanisms by which tissue injury is translated into inflammation and consequent fibrosis remain to be established. Here, we show that in a lung injury model, bleomycin induced the secretion of IL-6 by epithelial cells in a transglutaminase 2 (TG2)–dependent manner. This response represents a key step in the differentiation of IL-17–producing T cells and subsequent inflammatory amplification in the lung. The essential role of epithelial cells, but not inflammatory cells, TG2 was confirmed in bone marrow chimeras; chimeras made in TG2-deficient recipients showed reduced inflammation and fibrosis, compared with those in wild-type mice, regardless of the bone marrow cell phenotype. Epithelial TG2 thus appears to be a critical inducer of inflammation after noninfectious pulmonary injury. We further demonstrated that fibroblast-derived TG2, acting downstream of transforming growth factor-β, is also important in the effector phase of fibrogenesis. Therefore, TG2 represents an interesting potential target for therapeutic intervention.

Murine hepatic stellate cells veto CD8 T cell activation by a CD54-dependent mechanism

The liver has a role in T cell tolerance induction, which is mainly achieved through the functions of tolerogenic hepatic antigen-presenting cells (APCs) and regulatory T cells. Hepatic stellate cells (HSCs) are known to have various immune functions, which range from immunogenic antigen presentation to the induction of T cell apoptosis. Here we report a novel role for stellate cells in vetoing the priming of naive CD8 T cells. Murine and human HSCs and stromal cells (but not hepatocytes) prevented the activation of naive T cells by dendritic cells, artificial APCs, and phorbol 12-myristate 13-acetate/ionomycin by a cell contact-dependent mechanism. The veto function for inhibiting T cell activation was directly correlated with the activation state of HSCs and was most pronounced in HSCs from fibrotic livers. Mechanistically, high expression levels of CD54 simultaneously restricted the expression of interleukin-2 (IL-2) receptor and IL-2 in T cells, and this was responsible for the inhibitory effect because exogenous IL-2 overcame the HSC veto function.

CONCLUSION

Our results demonstrate a novel function of HSCs in the local skewing of immune responses in the liver through the prevention of local stimulation of naive T cells. These results not only indicate a beneficial role in hepatic fibrosis, for which increased CD54 expression on HSCs could attenuate further T cell activation, but also identify IL-2 as a key cytokine in mediating local T cell immunity to overcome hepatic tolerance.

Functional specializations of intestinal dendritic cell and macrophage subsets that control Th17 and regulatory T cell responses are dependent on the T cell/APC ratio, source of mouse strain, and regional localization

Although several subsets of intestinal APCs have been described, there has been no systematic evaluation of their phenotypes, functions, and regional localization to date. In this article, we used 10-color flow cytometry to define the major APC subsets in the small and large intestine lamina propria. Lamina propria APCs could be subdivided into CD11c(+)CD11b(-), CD11c(+)CD11b(+), and CD11c(dull)CD11b(+) subsets. CD11c(+)CD11b(-) cells were largely CD103(+)F4/80(-) dendritic cells (DCs), whereas the CD11c(+)CD11b(+) subset comprised CD11c(+)CD11b(+)CD103(+)F4/80(-) DCs and CD11c(+)CD11b(+)CD103(-)F4/80(+) macrophage-like cells. The majority of CD11c(dull)CD11b(+) cells were CD103(-)F4/80(+) macrophages. Although macrophages were more efficient at inducing Foxp3(+) regulatory T (T(reg)) cells than DCs, at higher T cell/APC ratios, all of the DC subsets efficiently induced Foxp3(+) T(reg) cells. In contrast, only CD11c(+)CD11b(+)CD103(+) DCs efficiently induced Th17 cells. Consistent with this, the regional distribution of CD11c(+)CD11b(+)CD103(+) DCs correlated with that of Th17 cells, with duodenum > jejunum > ileum > colon. Conversely, CD11c(+)CD11b(-)CD103(+) DCs, macrophages, and Foxp3(+) T(reg) cells were most abundant in the colon and scarce in the duodenum. Importantly, however, the ability of DC and macrophage subsets to induce Foxp3(+) T(reg) cells versus Th17 cells was strikingly dependent on the source of the mouse strain. Thus, DCs from C57BL/6 mice from Charles River Laboratories (that have segmented filamentous bacteria, which induce robust levels of Th17 cells in situ) were more efficient at inducing Th17 cells and less efficient at inducing Foxp3(+) T(reg) cells than DCs from B6 mice from The Jackson Laboratory. Thus, the functional specializations of APC subsets in the intestine are dependent on the T cell/APC ratio, regional localization, and source of the mouse strain.

Loss of CNS IL-2 gene expression modifies brain T lymphocyte trafficking: response of normal versus autoreactive Treg-deficient T cells

Emerging data from our lab and others suggested that dysregulation of the brain's endogenous neuroimmunological milieu may occur with the loss of brain IL-2 gene expression and be involved in initiating processes that lead to CNS autoimmunity. We sought to test our working hypothesis that IL-2 deficiency induces endogenous changes in the CNS that play a key role in eliciting T cell homing into the brain. To accomplish this goal, we used an experimental approach that combined mouse congenic breeding and immune reconstitution. In congenic mice without brain IL-2 (two IL-2 KO alleles) that were reconstituted with a normal wild-type immune system, the loss of brain IL-2 doubled the number of T cells that trafficked into the brain in all regions quantified (hippocampus, septum, and cerebellum) compared to mice with two wild-type brain IL-2 alleles and a wild-type peripheral immune system. Congenic mice with normal brain IL-2 (two wild-type IL-2 alleles) that were immune reconstituted with autoreactive Treg-deficient T cells from IL-2 KO mice developed the expected peripheral autoimmunity (splenomegaly) and had a comparable doubling of T cell trafficking into the hippocampus and septum, whereas they exhibited an additional twofold proclivity for the cerebellum over the septohippocampal regions. Unlike brain trafficking of wild-type T cells, the increased homing of IL-2 KO T cells to the cerebellum was independent of brain IL-2 gene expression. These findings demonstrate that brain IL-2 deficiency induces endogenous CNS changes that may lead to the development of brain autoimmunity, and that autoreactive Treg-deficient IL-2 KO T cells trafficking to the brain could have a proclivity to induce cerebellar neuropathology.

Most B cells in non-lymphoid tissues are naïve

The current view of lymphocyte migration states that naïve lymphocytes re-circulate between the blood and the lymph via the lymph nodes, but are not able to access non-lymphoid tissues. We examined B lymphocytes in peripheral tissues and found that the majority were phenotypically similar to naïve B cells in lymphoid tissues and were located within the parenchyma, not associated with blood vessels. The mutation rate within the Vh region of these cells was substantially less than the rate attributed to somatic hypermutation and was identical to that observed in naïve B cells isolated from the lymph nodes, showing the presence of naïve B cells in the non-lymphoid organs. Further, using FTY720-treated mice, we showed that naïve B cells migrate through the peripheral tissues and, using pertussis toxin, that the entry of B cells was not controlled by chemokine-mediated signalling events. Overall, these results show that naïve B lymphocytes constitute the majority of the total B-cell population in non-lymphoid tissues and suggest that these cells may re-circulate through the periphery as part of their normal migration pathway. This has implications for the current view of the role of naïve B cells in priming and tolerance.

Age and facial nerve axotomy-induced T cell trafficking: relation to microglial and motor neuron status

Following peripheral axotomy of the facial nerve in mice, T lymphocytes cross the blood-brain-barrier (BBB) into the central nervous system (CNS), where they home to the neuronal cell bodies of origin in the facial motor nucleus (FMN) and act in concert with microglial cells to support the injured motor neurons. Several lines of evidence suggested normal aging may alter the injury-related responses of T cells, microglia, and motor neurons in this model. In this study, we therefore sought to test the hypothesis that compared to 8-week-old mice (young adult), 52-week-old mice (advanced middle age) would exhibit more neuronal damage and increased T cell trafficking into the injured FMN following facial nerve resection. Comparison of 8- and 52-week-old mice at 7, 14, 21, and 28 days post-resection of the facial nerve, confirmed our hypothesis that age influences the kinetics of CD3(+) T lymphocyte trafficking in the axotomized FMN. The peak T cell response was significantly higher, occurred later, and remained elevated longer in the injured FMN of mice in the 52 week age group. Although the kinetics of motor neuron death (identified by quantifying CD11b(+) perineuronal microglial phagocytic clusters engulfing the dead neurons at 7, 14, 21, and 28 days post-resection) differed between the age groups, motor neuron profile counts at day 28 showed that levels of cumulative motor neuron loss did not differ between the age groups. Compared to 8-week-old mice, however, there was small reduction in the mean cell size of the surviving motor neurons in the 52 week age group. Since T lymphocyte function decreases with normal aging, it will be important to determine if increased T cell trafficking into the injured CNS is a compensatory response to the decreased function of older T cells, and if these and related neuroimmunological changes are more pronounced in mice in the late stages of the life cycle.

Memory T cells are enriched in lymph nodes of selectin-ligand-deficient mice

Fucosyltransferase-IV and -VII double knockout (FtDKO) mice reveal profound impairment in T cell trafficking to lymph nodes (LNs) due to an inability to synthesize selectin ligands. We observed an increase in the proportion of memory/effector (CD44(high)) T cells in LNs of FtDKO mice. We infected FtDKO mice with lymphocytic choriomeningitis virus to generate and track Ag-specific CD44(high)CD8 T cells in secondary lymphoid organs. Although frequencies were similar, total Ag-specific effector CD44(high)CD8 T cells were significantly reduced in LNs, but not blood, of FtDKO mice at day 8. In contrast, frequencies of Ag-specific memory CD44(high)CD8 T cells were up to 8-fold higher in LNs of FtDKO mice at day 60. Because wild-type mice treated with anti-CD62L treatment also showed increased frequencies of CD44(high) T cells in LNs, we hypothesized that memory T cells were preferentially retained in, or preferentially migrated to, FtDKO LNs. We analyzed T cell entry and egress in LNs using adoptive transfer of bone fide naive or memory T cells. Memory T cells were not retained longer in LNs compared with naive T cells; however, T cell exit slowed significantly as T cell numbers declined. Memory T cells were profoundly impaired in entering LNs of FtDKO mice; however, memory T cells exhibited greater homeostatic proliferation in FtDKO mice. These results suggest that memory T cells are enriched in LNs with T cell deficits by several mechanisms, including longer T cell retention and increased homeostatic proliferation.

Tumor masses support naive T cell infiltration, activation, and differentiation into effectors

Studies of T cell responses to tumors have focused on the draining lymph node (LN) as the site of activation. We examined the tumor mass as a potential site of activation after adoptive transfer of naive tumor-specific CD8 T cells. Activated CD8 T cells were present in tumors within 24 h of adoptive transfer and proliferation of these cells was also evident 4-5 d later in mice treated with FTY720 to prevent infiltration of cells activated in LNs. To confirm that activation of these T cells occurred in the tumor and not the tumor-draining LNs, we used mice lacking LNs. Activated and proliferating tumor-infiltrating lymphocytes were evident in these mice 24 h and 4 d after naive cell transfer. T cells activated within tumors acquired effector function that was evident both ex vivo and in vivo. Both cross-presenting antigen presenting cells within the tumor and tumor cells directly presenting antigen activated these functional CD8 effectors. We conclude that tumors support the infiltration, activation, and effector differentiation of naive CD8 T cells, despite the presence of immunosuppressive mechanisms. Thus, targeting of T cell activation to tumors may present a tool in the development of cancer immunotherapy.

Naïve T cells re-distribute to the lungs of selectin ligand deficient mice

Background

Selectin mediated tethering represents one of the earliest steps in T cell extravasation into lymph nodes via high endothelial venules and is dependent on the biosynthesis of sialyl Lewis X (sLe^x) ligands by several glycosyltransferases, including two fucosyltransferases, fucosyltransferase-IV and –VII. Selectin mediated binding also plays a key role in T cell entry to inflamed organs.

Methodology/Principal Findings

To understand how loss of selectin ligands (sLe^x) influences T cell migration to the lung, we examined fucosyltransferase-IV and –VII double knockout (FtDKO) mice. We discovered that FtDKO mice showed significant increases (∼5-fold) in numbers of naïve T cells in non-inflamed lung parenchyma with no evidence of induced bronchus-associated lymphoid tissue. In contrast, activated T cells were reduced in inflamed lungs of FtDKO mice following viral infection, consistent with the established role of selectin mediated T cell extravasation into inflamed lung. Adoptive transfer of T cells into FtDKO mice revealed impaired T cell entry to lymph nodes, but selective accumulation in non-lymphoid organs. Moreover, inhibition of T cell entry to the lymph nodes by blockade of L-selectin, or treatment of T cells with pertussis toxin to inhibit chemokine dependent G-coupled receptor signaling, also resulted in increased T cells in non-lymphoid organs. Conversely, inhibition of T cell egress from lymph nodes using FTY720 agonism of S1P1 impaired T cell migration into non-lymphoid organs.

Conclusions/Significance

Taken together, our results suggest that impaired T cell entry into lymph nodes via high endothelial venules due to genetic deficiency of selectin ligands results in the selective re-distribution and accumulation of T cells in non-lymphoid organs, and correlates with their increased frequency in the blood. Re-distribution of T cells into organs could potentially play a role in the initiation of T cell mediated organ diseases.

CCR7 regulates lymphocyte egress and recirculation through body cavities

T and B lymphocytes recirculate among blood, lymph, and extralymphoid tissues to ensure immune surveillance and the establishment of self-tolerance. The underlying mechanisms regulating homeostatic lymphocyte recirculation through body cavities are not fully understood. Here, we demonstrate that the homeostatic chemokine receptor CCR7 regulates homeostatic recirculation of lymphocytes through body cavities. CCR7 deficiency results in massive accumulation of CD4(+) and CD8(+) T cells and B-2 B cells in the peritoneal and pleural cavities. The increase in B-2 B and T lymphocytes is not associated with an altered maturation and/or activation status of these cells. Mechanistically, an increase in peritoneal lymphocyte numbers is caused by impaired egress of CCR7-deficient lymphocytes from body cavities. These results establish that CCR7 plays a crucial role in lymphocyte exit from the PerC.

Cutting edge: contribution of lung-resident T cell proliferation to the overall magnitude of the antigen-specific CD8 T cell response in the lungs following murine influenza virus infection

Following influenza virus infection, CD8 T cells encounter mature, Ag-bearing dendritic cells within the draining lymph nodes and undergo activation, programmed proliferation, and differentiation to effector cells before migrating to the lungs to mediate viral clearance. However, it remains unclear whether CD8 T cells continue their proliferation after arriving in the lungs. To address this question, we developed a novel, in vivo, dual-label system using intranasal CFSE and BrdU administration to identify virus-specific CD8 T cells that are actively undergoing cell division while in the lungs. With this technique we demonstrate that a high frequency of virus-specific CD8 T cells incorporate BrdU while in the lungs and that this lung-resident proliferation contributes significantly to the magnitude of the Ag-specific CD8 T cell response following influenza virus infection.

CCR7 deficiency exacerbates injury in acute nephritis due to aberrant localization of regulatory T cells

The homing of dendritic cells and T cells to secondary lymphoid organs requires chemokine receptor 7 (CCR7) expression on these cells. T cells mediate the pathogenesis of experimental accelerated nephrotoxic serum nephritis (NTS), including its suppression by regulatory T cells (Tregs), but the contribution of CCR7 to this disease is unknown. Here, we compared the development of NTS in CCR7-knockout (KO) and wild-type (WT) mice. Compared with WT mice, CCR7KO mice developed more severe disease with significantly more inflammatory cells infiltrating the kidney. These cells included FoxP3(+) Tregs, which were virtually absent from WT kidneys. The adoptive transfer of WT Tregs into CCR7KO mice at the time of immunization protected the recipients from disease; these cells homed to secondary lymphoid organs but not to kidneys. Conversely, adoptive transfer of CCR7KO Tregs into WT mice did not inhibit development of NTS. These data suggest that NTS can develop without CCR7 expression, but Treg-mediated disease suppression, which seems to occur in secondary lymphoid organs, requires CCR7.

High frequencies of virus-specific CD8+ T-cell precursors

A productive CD8(+) T-cell response to a viral infection requires rapid division and proliferation of virus-specific CD8(+) T cells. Tetramer-based enrichment assays have recently given estimates of the numbers of peptide-major histocompatibility complex-specific CD8(+) T cells in naïve mice, but precursor frequencies for entire viruses have been examined only by using in vitro limiting-dilution assays (LDAs). To examine CD8(+) T-cell precursor frequencies for whole viruses, we developed an in vivo LDA and found frequencies of naïve CD8(+) T-cell precursors of 1 in 1,444 for vaccinia virus (VV) ( approximately 13,850 VV-specific CD8(+) T cells per mouse) and 1 in 2,958 for lymphocytic choriomeningitis virus (LCMV) ( approximately 6,761 LCMV-specific CD8(+) T cells per mouse) in C57BL/6J mice. In mice immune to VV, the number of VV-specific precursors, not surprisingly, dramatically increased to 1 in 13 ( approximately 1,538,462 VV-specific CD8(+) T cells per mouse), consistent with estimates of VV-specific memory T cells. In contrast, precursor numbers for LCMV did not increase in VV-immune mice (1 in 4,562, with approximately 4,384 LCMV-specific CD8(+) T cells per VV-immune mouse). Using H-2D(b)-restricted LCMV GP33-specific P14-transgenic T cells, we found that, after donor T-cell take was accounted for, approximately every T cell transferred underwent a full proliferative expansion in response to LCMV infection. This high efficiency was also seen with memory populations, suggesting that most antigen-specific T cells will proliferate extensively at a limiting dilution in response to infections. These results show that frequencies of naïve and memory CD8(+) T cell precursors for whole viruses can be remarkably high.

The blood-brain barrier in neurodegenerative disease: a rhetorical perspective

Recent studies suggest that the function of the blood-brain barrier (BBB) is not static under normal physiologic conditions and is likely altered in neurodegenerative disease. Prevailing thinking about CNS function, and neurodegenerative disease in particular, is neurocentric excluding the impact of factors outside the CNS. This review challenges this perspective and discusses recent reports suggesting the involvement of peripheral factors including toxins and elements of adaptive immunity that may not only play a role in pathogenesis, but also progression of neurodegenerative diseases. Central to this view is neuroinflammation. Several studies indicate that the neuroinflammatory changes that accompany neurodegeneration affect the BBB or its function by altering transport systems, enhancing immune cell entry, or influencing the BBB's role as a signaling interface. Such changes impair the BBB's normal homeostatic function and affect neural activity. Moreover, recent studies reveal that alterations in BBB and its transporters affect the entry of drugs used to treat neurodegenerative diseases. Incorporating BBB compromise and dysfunction into our view of neurodegenerative disease leads to the inclusion of peripheral mediators in its pathogenesis and progression. In addition, this changing view of the BBB raises interesting new therapeutic possibilities for drug delivery as well as treatment strategies designed to reinstate normal barrier function.

Aberrant tissue localization of fungus-specific CD4+ T cells in IL-10-deficient mice

Aspergillus fumigatus, a common environmental fungus, can cause lethal invasive infections in immunocompromised hosts. In immunocompetent individuals, however, inhaled A. fumigatus spores prime CD4(+) T cells and activate immune responses that prevent invasive infection. Calibration of inflammatory responses to levels that prevent fungal invasion without inducing collateral tissue damage is essential for host survival, but the underlying regulatory mechanisms remain undefined. Although IL-10 is a validated regulatory cytokine that suppresses immune responses, and IL-10 deficiency or blockade generally enhances immune responses, we find that A. fumigatus-specific T cell frequencies are markedly reduced in airways of IL-10-deficient mice. T cell priming, proliferation, and survival were unaffected by IL-10 deficiency and did not account for decreased frequencies of A. fumigatus-specific T cells in the airways of IL-10-deficient mice. Instead, IL-10 deficiency results in redistribution of A. fumigatus-specific T cells from infected lungs to the gut, a process that is reversed by antibiotic-mediated depletion of intestinal microbes. Our studies demonstrate that disregulated immune responses in the gut can result in dramatic redistribution of pathogen-specific T cells within the host.

Unconventional antigen-presenting cells in the induction of peripheral CD8(+) T cell tolerance

Bone marrow-derived APCs are considered the predominant cell type involved in the induction and maintenance of T cell tolerance in vivo. In the periphery, cross-presentation of self-antigens by DCs, in particular, CD8alpha(+) DCs, has been the most discussed mechanism underlying the induction of CD8(+) T cell tolerance against self. However, nonhematopoietic APCs in the liver, skin, parenchymal tissues, and lymph nodes can also present self- and exogenous antigens to CD8(+) T cells under steady-state conditions. Although far surpassed by their DC counterparts in their ability to stimulate T cell responses, these unconventional APCs have been shown to play a role in the induction, maintenance, and regulation of peripheral CD8(+) T cell tolerance by a multitude of mechanisms. In this review, we will discuss the different nonhematopoietic cells that have been shown to present tissue-specific or exogenous antigens to naïve CD8(+) T cells, thereby contributing to the regulation of T cell responses in the periphery.

Donor CD4 T cells contribute to cardiac allograft vasculopathy by providing help for autoantibody production

BACKGROUND

The development of autoantibody after heart transplantation is increasingly associated with poor graft outcome, but what triggers its development and whether it has a direct causative role in graft rejection is not clear. Here, we study the development of antinuclear autoantibody in an established mouse model of heart allograft vasculopathy.

METHODS AND RESULTS

Humoral vascular changes, including endothelial complement staining, were present in bm12 heart grafts, explanted 50 days after transplantation. Alloantibody was not detectable, but long-lasting autoantibody responses developed in C57BL/6 recipients from the third week after transplantation. No autoantibody was generated if donor CD4 T cells were depleted before heart graft retrieval or in recipients that lacked B-cell major histocompatibility complex class II expression, indicating that humoral autoimmunity is a consequence of donor CD4 T-cell allorecognition of the major histocompatibility complex class II complex on recipient autoreactive B cells. An effector role for autoantibody in graft rejection was confirmed by abrogation of humoral vascular rejection, and attenuation of vasculopathy, in B-cell deficient recipients and by development of vascular obliteration and accelerated rejection in recipients primed for autoantibody before transplantation.

CONCLUSIONS

Passenger CD4 T cells within heart transplants can contribute to allograft vasculopathy by providing help to recipient B cells for autoantibody generation.

Adenosine regulates CD8 T-cell priming by inhibition of membrane-proximal T-cell receptor signalling

Adenosine is a well-described anti-inflammatory modulator of immune responses within peripheral tissues. Extracellular adenosine accumulates in inflamed and damaged tissues and inhibits the effector functions of various immune cell populations, including CD8 T cells. However, it remains unclear whether extracellular adenosine also regulates the initial activation of naïve CD8 T cells by professional and semi-professional antigen-presenting cells, which determines their differentiation into effector or tolerant CD8 T cells, respectively. We show that adenosine inhibited the initial activation of murine naïve CD8 T cells after alphaCD3/CD28-mediated stimulation. Adenosine caused inhibition of activation, cytokine production, metabolic activity, proliferation and ultimately effector differentiation of naïve CD8 T cells. Remarkably, adenosine interfered efficiently with CD8 T-cell priming by professional antigen-presenting cells (dendritic cells) and semi-professional antigen-presenting cells (liver sinusoidal endothelial cells). Further analysis of the underlying mechanisms demonstrated that adenosine prevented rapid tyrosine phosphorylation of the key kinase ZAP-70 as well as Akt and ERK1/2 in naïve alphaCD3/CD28-stimulated CD8 cells. Consequently, alphaCD3/CD28-induced calcium-influx into CD8 cells was reduced by exposure to adenosine. Our results support the notion that extracellular adenosine controls membrane-proximal T-cell receptor signalling and thereby also differentiation of naïve CD8 T cells. These data raise the possibility that extracellular adenosine has a physiological role in the regulation of CD8 T-cell priming and differentiation in peripheral organs.

Infiltration of CD4+ lymphocytes into the brain contributes to neurodegeneration in a mouse model of Parkinson disease

Parkinson disease (PD) is a neurodegenerative disorder characterized by a loss of dopamine-containing neurons. Mounting evidence suggests that dopaminergic cell death is influenced by the innate immune system. However, the pathogenic role of the adaptive immune system in PD remains enigmatic. Here we showed that CD8+ and CD4+ T cells but not B cells had invaded the brain in both postmortem human PD specimens and in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of PD during the course of neuronal degeneration. We further demonstrated that MPTP-induced dopaminergic cell death was markedly attenuated in the absence of mature T lymphocytes in 2 different immunodeficient mouse strains (Rag1-/- and Tcrb-/- mice). Importantly, similar attenuation of MPTP-induced dopaminergic cell death was seen in mice lacking CD4 as well as in Rag1-/- mice reconstituted with FasL-deficient splenocytes. However, mice lacking CD8 and Rag1-/- mice reconstituted with IFN-gamma-deficient splenocytes were not protected. These data indicate that T cell-mediated dopaminergic toxicity is almost exclusively arbitrated by CD4+ T cells and requires the expression of FasL but not IFNgamma. Further, our data may provide a rationale for targeting the adaptive arm of the immune system as a therapeutic strategy in PD.

Re-entry of mature T cells to the thymus: an epiphenomenon?

In certain situations mature post-thymic T cells are able to leave their residence in the secondary lymphoid tissues and re-enter the thymus. The physiological significance of this phenomenon is discussed.

Form follows function: lymphoid tissue microarchitecture in antimicrobial immune defence

Secondary lymphoid organs (SLOs) are tissues that facilitate the induction of adaptive immune responses. These organs capture pathogens to limit their spread throughout the body, bring antigen-presenting cells into productive contact with their cognate lymphocytes and provide niches for the differentiation of immune effector cells. Therefore, the microanatomy of SLOs defines the ability of an organism to respond to pathogens. SLO microarchitecture is, at the same time, extremely adaptable to environmental changes. In this Review, we discuss recent insights into the function and plasticity of the SLO microenvironment with regards to antimicrobial immune defence.

T lymphocyte migration to lymph nodes is maintained during homeostatic proliferation

The immune system maintains appropriate cell numbers through regulation of cell proliferation and death. Normal tissue distribution of lymphocytes is maintained through expression of specific adhesion molecules and chemokine receptors such as L-selectin and CCR7, respectively. Lymphocyte insufficiency or lymphopenia induces homeostatic proliferation of existing lymphocytes to increase cell numbers. Interestingly, homeostatic proliferation of T lymphocytes induces a phenotypic change from naïve- to memory-type cell. Naïve T cells recirculate between blood and lymphoid tissues whereas memory T cells migrate to nonlymphoid sites such as skin and gut. To assess effects of homeostatic proliferation on migratory ability of T cells, a murine model of lymphopenia-induced homeostatic proliferation was used. Carboxyfluorescein diacetate, succinimidyl ester-labeled wild-type splenocytes were adoptively transferred into recombination activation gene-1-deficient mice and analyzed by flow cytometry, in vitro chemotactic and in vivo migration assays, and immunofluorescence microscopy. Homeostatically proliferated T cells acquired a mixed memory-type CD44high L-selectinhigh CCR7low phenotype. Consistent with this, chemotaxis to secondary lymphoid tissue chemokine in vitro was reduced by 22%-34%. By contrast, no differences were found for migration or entry into lymph nodes during in vivo migration assays. Therefore, T lymphocytes that have undergone homeostatic proliferation recirculate using mechanisms similar to naïve T cells.

Endogenous naive CD8+ T cell precursor frequency regulates primary and memory responses to infection

Through genetic recombination, the adaptive immune system generates a diverse T cell repertoire allowing recognition of a vast spectrum of foreign antigens. Any given CD8+ T cell specificity is thought to be rare, but none have been directly quantified. Here, major histocompatibility complex tetramer and magnetic-bead technology were coupled to quantitate naive antigen-specific CD8+ T cells and the early response to infection. Among six specificities measured, the number of naive antigen-specific precursors ranged from approximately 80 to 1200 cells/mouse. After vesicular stomatitis virus infection, the antigen-specific CD8+ T cell response occurred in discrete phases: prolonged activation of a subset of cells over the first 72 hr followed by a rapid proliferative burst. Naive precursor frequency altered response kinetics and regulated immunodominance, as well as the time required for the responding population to shift toward CD62L(hi) memory cells. Thus, initial endogenous precursor frequencies were surprisingly diverse and not only regulated initial immune response characteristics but also controlled memory CD8+ T cell lineage decisions.

Geography and plumbing control the T cell response to infection

The orchestrated movement of cells of the immune system is essential to generation of productive responses leading to protective memory development. Recent advances have allowed the direct microscopic visualization of lymphocyte and antigen-presenting cell migration and interaction during immune response initiation and progression. These studies have defined important characteristics of the microanatomy of lymphocyte movement, particularly in the lymph node. Moreover, the ability to track endogenous antigen-specific T cells has revealed a coordinated pathway of CD8 T cell movement in the spleen following primary and secondary infection. As a consequence, the local anatomy of secondary lymphoid tissues during infection has emerged as a critical regulator of immunity. While some of the factors responsible for the migratory cues instructing immune cell movement have been identified, much remains to be learned. Here, we provide a brief overview of studies examining CD8 T cell localization during the immune response to infection in the context of our current understanding of immune system structure.

Requirement of secondary lymphoid tissues for the induction of primary and secondary T cell responses against Listeria monocytogenes

Activation of naive T cells is tightly controlled and depends on cognate interactions with professional antigen-presenting cells. We analyzed dependency on secondary lymphoid tissues for the activation of naive and memory CD4(+) and CD8(+) T cells following primary and secondary Listeria monocytogenes infection, respectively. In splenectomized lymphotoxin-beta receptor-deficient mice, lacking all secondary lymphoid tissues, oral infection with L. monocytogenes failed to induce bacteria-specific CD4(+) and CD8(+) T cell responses. Treatment of splenectomized wild-type mice with FTY720, a drug that prevents egress of T cells from lymph nodes, also reduced T cell responses after oral L. monocytogenes infection and blocked T cell responses after intravenous infection. FTY720-treated wild-type and lymphotoxin-beta receptor-deficient mice show only slightly impaired recall responses. However, T cell responses were profoundly inhibited when mice were splenectomized subsequently to recovery from primary infection. T cell transfer experiments demonstrated that the impaired secondary T cell response was not simply due to removal of a large fraction of memory T cells by splenectomy. Overall, these results indicate that not only primary T cell responses, but also secondary T cell responses, highly depend on the lymphoid environment for effective activation.

Alveolar epithelial type II cells induce T cell tolerance to specific antigen

The lungs face the immunologic challenge of rapidly eliminating inhaled pathogens while maintaining tolerance to innocuous Ags. A break in this immune homeostasis may result in pulmonary inflammatory diseases, such as allergies or asthma. The observation that alveolar epithelial type II cells (Type II) constitutively express the class II MHC led us to hypothesize that Type II cells play a role in the adaptive immune response. Because Type II cells do not express detectable levels of the costimulatory molecules CD80 and CD86, we propose that Type II cells suppress activation of naive T cells. Purified murine Type II cells were unable to activate T cells to specific Ag or in an alloreactive assay. Although IFN-gamma treatment up-regulated class II MHC expression, it did not alter the ability of the Type II cells to activate T cells. Rather, the Type II cells were able to suppress T cells from subsequent activation to specific Ag in an Ag-dependent manner. Priming T cells with Type II cells and Ag resulted in T cells that were suppressed to further activation, even after removal from the Type II cells. Thus, Type II cells of the lung help tolerate T cells to nonpathogenic environmental Ags.

Quantitative analysis of T cell homeostatic proliferation

T cell homeostatic proliferation occurs on transfer of T cells into lymphopenic recipients; transferred cells undergo several rounds of division in the absence of specific antigen stimulation. For a quantitative analysis of this phenomenon, we applied a mathematical method to describe proliferating T cells to match peak distributions from actual CFSE dilution data. For in vitro stimulation of T cells with anti-CD3/anti-CD28, our simulation confirmed a high proportion of cells entering cell cycle with a low proportion undergoing apoptosis. When applied to homeostatic proliferation, it described striking differences in CD4 and CD8 T cell proliferation rates, and accurately predicted that successive divisions were accompanied by higher rates of apoptosis, limiting the accumulation of proliferating cells. Thus, the presence of multiple CFSE dilution peaks cannot be considered equivalent to lymphocyte expansion. Finally, genetic effects were identified that may help explain links between homeostatic proliferation and autoimmunity.

Homing imprinting and immunomodulation in the gut: role of dendritic cells and retinoids

Lymphocyte migration is at the heart of chronic inflammatory ailments, including inflammatory bowel disease (IBD). Whereas naïve lymphocytes migrate to all secondary lymphoid organs, they are mostly excluded from nonlymphoid peripheral tissues. Upon activation, lymphocytes change their pattern of adhesion receptors and acquire the capacity to migrate to extralymphoid tissues. Antigen-experienced T cells are subdivided into different subsets based on their expression of homing receptors that favor their accumulation in specific tissues, such as the skin and the gut mucosa. B cells and antibody-secreting cells (ASC) also show tissue-tropism, which is somewhat correlated with the class of immunoglobulin that they produce. In fact, IgA-ASC are located in mucosal tissues, where they produce IgA, the main class of antibodies found in secretions. Although IgA-ASC are usually considered as a homogeneous pool of cells, those located in the small bowel have some unique migratory characteristics, suggesting that they are generated under different conditions as compared to IgA-ASC in other mucosal compartments. Foxp3(+) regulatory T cells (T(REG)) can also exhibit tissue-specific migratory potential and recent evidence suggests that T(REG) can be imprinted with gut-specific homing. Moreover, foxp3(+) T(REG) are enriched in the small bowel lamina propria, where they can be generated locally. The present review addresses our current understanding of how tissue-specific homing is acquired and modulated on T cells, B cells, and ASC, with a special emphasis on the intestinal mucosa. Harnessing these mechanisms could offer novel, effective, and more specific therapeutic strategies in IBD.

Modulation of pulmonary dendritic cell function during mycobacterial infection

We have previously reported that during mycobacterial infection, naïve CD4(+) T-cell activation is enhanced in the lungs. We investigated the role of chemokine receptor CCR7 and its ligands in the ability of CD11c(+) lung dendritic cells (DCs) to activate naïve CD4(+) T cells during pulmonary infection with Mycobacterium bovis bacillus Calmette-Guérin (BCG). BCG infection resulted in the accumulation and maturation in the lungs of DCs that persisted as the mycobacterial burden declined. Lung DCs from infected mice expressed more major histocompatibility complex class II (MHC-II) than those from uninfected mice. CCR7 expression levels on lung DCs were comparable among uninfected and infected mice. The gene expression of the CCR7 ligand CCL19 progressively increased throughout BCG infection, and its expression was MyD88 dependent. CD11c(+) lung cells from BCG-infected mice activated ovalbumin (OVA)-specific naïve CD4(+) T cells more than CD11c(+) lung cells from uninfected mice. Interestingly, during peak mycobacterial infection, CD11c(hi) MHC(hi) lung DCs had slightly decreased chemotaxis toward the CCR7 ligand CCL21 and less efficiency in activating naive CD4(+) T cells than DCs from mice during late-stage infection, when few bacilli are found in the lung. These findings suggest that during BCG infection, the inflammation and sustained expression of CCL19 result in the recruitment, activation, and retention in the lung of DCs that can activate naïve CD4(+) T cells in situ.

Memory versus naive T-cell migration

Our established understanding of lymphocyte migration suggests that naive and memory T cells travel throughout the body via divergent pathways; naive T cells circulate between blood and lymph whereas memory T cells additionally migrate through non-lymphoid organs. Evidence is now gradually emerging which suggests such disparate pathways between naive and memory T cells may not strictly be true, and that naive T cells gain access to the non-lymphoid environment in numbers approaching that of memory T cells. We discuss here the evidence for naive T-cell traffic into the non-lymphoid environment, compare and contrast this movement with what is known of memory T cells, and finally discuss the functional importance of why naive T cells might access the parenchymal tissues.