Canine CD4(+)CD8(+) double-positive T cells can develop from CD4(+) and CD8(+) T cells

A review of CD4+ T cell differentiation and diversity in dogs

CD4+ T cells are an integral component of the adaptive immune response, carrying out many functions to combat a diverse range of pathogenic challenges. These cells exhibit remarkable plasticity, differentiating into specialized subsets such as T helper type 1 (TH1), TH2, TH9, TH17, TH22, regulatory T cells (Tregs), and follicular T helper (TFH) cells. Each subset is capable of addressing a distinct immunological need ranging from pathogen eradication to regulation of immune homeostasis. As the immune response subsides, CD4+ T cells rest down into long-lived memory phenotypes-including central memory (TCM), effector memory (TEM), resident memory (TRM), and terminally differentiated effector memory cells (TEMRA) that are localized to facilitate a swift and potent response upon antigen re-encounter. This capacity for long-term immunological memory and rapid reactivation upon secondary exposure highlights the role CD4+ T cells play in sustaining both adaptive defense mechanisms and maintenance. Decades of mouse, human, and to a lesser extent, pig T cell research has provided the framework for understanding the role of CD4+ T cells in immune responses, but these model systems do not always mimic each other. Although our understanding of pig immunology is not as extensive as mouse or human research, we have gained valuable insight by studying this model. More akin to pigs, our understanding of CD4+ T cells in dogs is much less complete. This disparity exists in part because canine immunologists depend on paradigms from mouse and human studies to characterize CD4+ T cells in dogs, with a fraction of available lineage-defining antibody markers. Despite this, every major CD4+ T cell subset has been described to some extent in dogs. These subsets have been studied in various contexts, including in vitro stimulation, homeostatic conditions, and across a range of disease states. Canine CD4+ T cells have been categorized according to lineage-defining characteristics, trafficking patterns, and what cytokines they produce upon stimulation. This review addresses our current understanding of canine CD4+ T cells from a comparative perspective by highlighting both the similarities and differences from mouse, human, and pig CD4+ T cell biology. We also discuss knowledge gaps in our current understanding of CD4+ T cells in dogs that could provide direction for future studies in the field.

Peripheral blood lymphocyte subtypes in dogs with different stages of myxomatous mitral valve disease

Background

Data on alterations in peripheral blood lymphocyte (PBL) subtypes in dogs with myxomatous mitral valve disease (MMVD) is lacking.

Objectives

To investigate PBL subtypes and their correlation with parameters of inflammation and MMVD progression markers in dogs with different stages of MMVD.

Animals

Seventy‐eight client‐owned dogs: 65 with MMVD (American College of Veterinary Internal Medicine [ACVIM] classification stages B2, C, and D) and 13 healthy controls.

Methods

Prospective cross‐sectional study. Complete cardiac assessment, flow cytometry (T lymphocytes [CD3+], their subtypes [CD3+CD4+, CD3+CD8+, CD3+CD4+CD8+, CD3+CD4−CD8−], and B lymphocytes [CD45+CD21+]) and measurement of N‐terminal pro B‐type natriuretic peptide, cardiac troponin I, and C‐reactive protein concentrations were performed.

Results

The percentage of CD3+CD4+ lymphocytes was significantly lower in stable ACVIM C patients (P = .01) and unstable ACVIM C and D patients (P = .003), the percentage of CD3+CD8+ lymphocytes was significantly higher in stable ACVIM C patients (P = .01) and unstable ACVIM C and D patients (P = .01), CD3+CD8+ lymphocyte concentration was significantly higher in unstable ACVIM C and D patients (P = .05), and the CD3+CD4+/CD3+CD8+ ratio was significantly lower in stable ACVIM C patients (P = .01) and unstable ACVIM C and D patients (P = .01) compared with healthy controls.

Conclusions and Clinical Importance

The percentages of CD3+CD4+ and CD3+CD8+ PBL and CD4+/CD8+ ratio were altered in MMVD dogs with congestive heart failure (ACVIM C, D), but not in ACVIM B2, suggesting involvement of these PBL subtypes in the pathogenesis of congestive heart failure in dogs with MMVD.

Food allergen-specific sublingual immunotherapy modulates peripheral T cell responses of dogs with adverse food reactions

Food allergen-specific sublingual immunotherapy (FA-SLIT) is a novel, safe and effective approach in dogs with adverse food reactions (AFR) to reduce their clinical symptoms. However, little is known about the specific immune components which mediate this reduction in clinical symptoms. In humans, regulatory T cells seem to play an important role in this desensitisation process. Here, we investigated changes in peripheral T cell responses of dogs with AFR upon FA-SLIT. Five dogs received a dose escalation of FA-SLIT over a six-month period. An oral food challenge was performed at the beginning and end of the study to assess the efficacy of the FA-SLIT. Using in vitro allergen-recall assays, we assessed the proliferation of T cell subsets before and at the end of the treatment. FA-SLIT significantly increased the percentage of proliferating CD4-CD8- double-negative (DN) T cells, while the percentage of allergen-specific CD4-CD8+ and CD4+CD8+ double-positive (DP) T cells decreased upon treatment. These findings indicate that sublingual immunotherapy in dogs activates DN T cells, which might be important for the desensitisation of dogs with adverse food reactions. However, further research is needed to corroborate these findings and to further elucidate the mechanism of action of FA-SLIT in dogs with AFR.

Canine tissue-associated CD4+CD8α+ double-positive T cells are an activated T cell subpopulation with heterogeneous functional potential

Canine CD4⁺CD8α⁺ double-positive (dp) T cells of peripheral blood are a unique effector memory T cell subpopulation characterized by an increased expression of activation markers in comparison with conventional CD4⁺ or CD8α⁺ single-positive (sp) T cells. In this study, we investigated CD4⁺CD8α⁺ dp T cells in secondary lymphatic organs (i.e. mesenteric and tracheobronchial lymph nodes, spleen, Peyer’s patches) and non-lymphatic tissues (i.e. lung and epithelium of the small intestine) within a homogeneous group of healthy Beagle dogs by multi-color flow cytometry. The aim of this systematic analysis was to identify the tissue-specific localization and characteristics of this distinct T cell subpopulation. Our results revealed a mature extrathymic CD1a^-CD4⁺CD8α⁺ dp T cell population in all analyzed organs, with highest frequencies within Peyer’s patches. Constitutive expression of the activation marker CD25 is a feature of many CD4⁺CD8α⁺ dp T cells independent of their localization and points to an effector phenotype. A proportion of lymph node CD4⁺CD8α⁺ dp T cells is FoxP3⁺ indicating regulatory potential. Within the intestinal environment, the cytotoxic marker granzyme B is expressed by CD4⁺CD8α⁺ dp intraepithelial lymphocytes. In addition, a fraction of CD4⁺CD8α⁺ dp intraepithelial lymphocytes and of mesenteric lymph node CD4⁺CD8α⁺ dp T cells is TCRγδ⁺. However, the main T cell receptor of all tissue-associated CD4⁺CD8α⁺ dp T cells could be identified as TCRαβ. Interestingly, the majority of the CD4⁺CD8α⁺ dp T cell subpopulation expresses the unconventional CD8αα homodimer, in contrast to CD8α⁺ sp T cells, and CD4⁺CD8α⁺ dp thymocytes which are mainly CD8αβ⁺. The presented data provide the basis for a functional analysis of tissue-specific CD4⁺CD8α⁺ dp T cells to elucidate their role in health and disease of dogs.

Antigen-Specific CD4+CD8+ Double-Positive T Cells Are Increased in the Blood and Spleen During Ehrlichia chaffeensis Infection in the Canine Host

Ehrlichia chaffeensis is an obligate intracellular bacterium belonging to the order, Rickettsiales and is a frequent cause of severe and fatal tick-borne infection in people in North America. The reservoir host for E. chaffeensis is the white-tailed deer, while humans and dogs are regarded as common incidental hosts. In dogs, we and others have shown that E. chaffeensis establishes a chronic infection that persists for several weeks to months, while promoting the development of Th1 and Th17 cellular responses and pathogen-specific humoral immunity. We demonstrate here that vaccination with a live, attenuated clone of E. chaffeensis bearing a targeted mutation in the Ech_0230 gene neither promotes the development of long-lived cellular or humoral immunity, nor confers protection against secondary wild-type E. chaffeensis challenge. In dogs, a population of mature CD4+CD8+ double-positive (DP) T cells exists in the periphery that shares similarities with the DP T cell populations that have been described in humans and swine. Little is known about the function of these cells, particularly in the context of infectious diseases. Here, we demonstrate that canine DP T cells expand significantly in response to E. chaffeensis infection. Using in vitro antigen recall assays, we further demonstrate that canine DP T cells undergo clonal expansion, produce IFNγ and IL-17, and upregulate expression of granzyme B and granulysin. Together, our results demonstrate that DP T cells accumulate in the host during E. chaffeensis infection, and suggest that alternative lymphocyte populations may participate in the immune response to tick-borne infections in the incidental host.

Zoonotic intestinal helminths interact with the canine immune system by modulating T cell responses and preventing dendritic cell maturation

Parasite co-evolution alongside the mammalian immune system gave rise to several modulatory strategies by which they prevent exaggerated pathology and facilitate a longer worm survival. As little is known about the immunoregulatory potential of the zoonotic canine parasites Ancylostoma caninum and Toxocara canis in the natural host, the present study aimed to investigate whether their larval excretory-secretory (ES) products can modulate the canine immune system. We demonstrated TcES to increase the frequency of CD4+ Foxp3^high T cells, while both AcES and TcES were associated with elevated Helios expression in Foxp3^high lymphocytes. ES products were further capable of inducing IL-10 production by lymphocytes, which was mainly attributed to CD8+ T cells. ES treatment of PBMCs prior to mitogen stimulation inhibited polyclonal proliferation of CD4+ and CD8+ T cells. Moreover, monocyte-derived ES-pulsed dendritic cells reduced upregulation of MHC-II and CD80 in response to lipopolysaccharide. The data showed that regulation of the canine immune system by A. caninum and T. canis larvae comprises the modification of antigen-specific and polyclonal T cell responses and dendritic cell maturation.

CD4+CD8β+ double-positive T cells in skin-draining lymph nodes respond to inflammatory signals from the skin

CD4⁺CD8⁺ double-positive (DP), mature, peripheral T cells are readily detectable in a variety of species and tissues. Despite a common association with autoimmune and malignant skin disorders, however, little is understood about their role or function. Herein, we show that DP T cells are readily detectable in the blood, spleen, and peripheral lymph nodes of naïve C57BL/6 mice. DP T cells were also present in Jα18^-/- and CD1d^-/- mice, indicating that these cells are not NK-T cells. After skin administration of CASAC adjuvant, but not Quil A adjuvant, both total DP T cells and skin-infiltrating DP T cells increased in number. We explored the possibility that DP T cells could represent aggregates between CD4⁺ and CD8⁺ single-positive T cells and found strong evidence that a large proportion of apparent DP T cells were indeed aggregates. However, the existence of true CD4⁺CD8⁺ DP T cells was confirmed by Amnis ImageStream (Millipore Sigma, Billerica, MA, USA) imaging. Multiple rounds of FACS sorting separated true DP cells from aggregates and indicated that conventional analyses may lead to ∼10-fold overestimation of DP T cell numbers. The high degree of aggregate contamination and overestimation of DP abundance using conventional analysis techniques may explain discrepancies reported in the literature for DP T cell origin, phenotype, and function.

T lymphocyte immunophenotypes in the cerebrospinal fluid of dogs with visceral leishmaniasis

Visceral leishmaniasis (VL) is a disease causing several clinical manifestations in dogs, including neurological disorders. Nevertheless, there are few studies related to the evaluation of the brain alterations during VL. Evidences of the involvement of cerebral barriers in infected dogs was reported, including the presence of brain inflammatory infiltrate, with a predominance of CD3+ T cells. Therefore, the aim of this study was to determine the immunophenotypes of T lymphocytes in the cerebrospinal fluid (CSF), as well as in peripheral blood, and to correlate with brain alterations in dogs with VL. We detected elevated percentages of double negative (DN) and double positive (DP) T cells in the CSF, with a predominance of TCRαb. In the histopathological analysis, we observed a predominance of lymphoplasmacytic infiltrate, mainly in leptomeninges, ranging from mild to intense, and we observed a positive correlation between the intensity of inflammation in the subependymal area and the DN T cells of the CSF. Thus, the DN T cells seem be acting as villains of the immune system through pro-inflammatory mechanisms. Further, the proportion of the different population of CSF T cells did not differ from those observed in the blood, which provides us with more evidence of blood-CSF barrier breakdown. Together, the results provide more explanation to the inflammation observed in the brain of dogs with VL, which the DN T cells contribute to the origin and progression of the neurological disease. This study provides insight into the immunophenotypes of T lymphocytes in the CSF during canine visceral leishmaniasis.

Peripheral canine CD4(+)CD8(+) double-positive T cells - unique amongst others

T lymphocytes co-expressing CD4 and CD8 ("double-positive T cells") are commonly associated with a thymic developmental stage of T cells. Their first description in humans and pigs as extrathymic T cells with a memory phenotype almost 30 years ago came as a surprise. Meanwhile peripheral double-positive T cells have been described in a growing number of different species. In this review we highlight novel data from our very recent studies on canine peripheral double-positive T cells which point to unique features of double-positive T cells in the dog. In contrast to porcine CD4(+)CD8(+) T cells forming a homogenous cellular population based on their expression of CD4 and CD8α, canine CD4(+)CD8(+) T cells can be divided into three different cellular subsets with distinct expression levels of CD4 and CD8α. Double-positive T cells expressing CD8β are present in humans and dogs but absent in swine. Moreover, canine CD4(+)CD8(+) T cells can not only develop from CD4(+) single-positive T cells but also from CD8(+) single-positive T cells. Together, this places canine CD4(+)CD8(+) T cells closer to their human than porcine counterparts since human double-positive T cells also appear to be heterogeneous in their CD4 and CD8α expression and have both CD4(+) and CD8(+) T cells as progenitor cells. However, CD4(+) single-positive T cells are the more potent progenitors for canine double-positive T cells, whereas CD8(+) single-positive T cells are more potent progenitors for human double-positive T cells. Canine double-positive T cells have an activated phenotype and may have as yet unrecognized roles in vivo in immunity to infection or in inflammatory diseases such as chronic infection, autoimmunity, allergy, or cancer.