Sex differences in the immune response to experimental stroke: Implications for translational research

Ischemic stroke is a leading cause of death and disability in the United States. It is known that males and females respond differently to stroke. Depending on age, the incidence, prevalence, mortality rate, and disability outcome of stroke differ between the sexes. Females generally have strokes at older ages than males and, therefore, have a worse stroke outcome. There are also major differences in how the sexes respond to stroke at the cellular level. Immune response is a critical factor in determining the progress of neurodegeneration after stroke and is fundamentally different for males and females. Additionally, females respond to stroke therapies differently from males, yet they are often left out of the basic research that is focused on developing those therapies. With a resounding failure to translate stroke therapies from the bench to the bedside, it is clearer than ever that inclusion of both sexes in stroke studies is essential for future clinical success. This Mini-Review examines sex differences in the immune response to experimental stroke and its implications for therapy development. © 2016 Wiley Periodicals, Inc.

Sex differences and the role of PPAR alpha in experimental stroke

Males and females respond differently to stroke. Moreover, females often experience worse long-term stroke outcomes. Fenofibrate, a peroxisome proliferator-activated receptor alpha (PPARα) agonist has been shown to improve stroke outcome and resolve neuroinflammation in male mice. The present study compares the effect of pretreatment with fenofibrate versus vehicle control in male and female mice during experimental stroke. Mice were treated with low-dose fenofibrate 30 min before and once a day for three additional days after stroke onset. We observed a reduction in infarct volume in male mice 96 h post-stroke with low-dose fenofibrate pretreatment that was due to increase of an M2 macrophage phenotype in the brain and an increase in regulatory cells in the periphery. These outcomes were not replicated in females, likely due to the lower PPARα expression in cells and tissues in females vs males. We conclude that PPARα agonist treatment prior to stroke is neuroprotective in males but not females. These findings indicate PPARα as a probable mechanism of sex difference in stroke outcome and support the need for representation of females in stroke therapy research.

Loss of PPARα perpetuates sex differences in stroke reflected by peripheral immune mechanisms

Peroxisome proliferator-activated receptor alpha (PPARα) is a nuclear receptor transcription factor that plays a role in immune regulation. Because of its expression in cerebral tissue and immune cells, PPARα has been examined as an important regulator in immune-based neurological diseases. Many studies have indicated that pre-treatment of animals with PPARα agonists induces protection against stroke. However, our previous reports indicate that protection is only in males, not females, and can be attributed to different PPARα expression between the sexes. In the current study, we examine how loss of PPARα affects male and female mice in experimental stroke. Male and female PPARα knockout mice were subject to middle cerebral artery occlusion (MCAO) or sham surgery, and the ischemic (local) or spleen specific (peripheral) immune response was examined 96 h after reperfusion. We found that loss of PPARα perpetuated sex differences in stroke, and this was driven by the peripheral, not local, immune response. Specifically we observed an increase in peripheral pro-inflammatory and adhesion molecule gene expression in PPARα KO males after MCAO compared to females. Our data supports previous evidence that PPARα plays an important role in sex differences in the immune response to disease, including stroke.

Adoptive transfer of immune subsets prior to MCAO does not exacerbate stroke outcome in splenectomized mice

The peripheral immune response contributes to neurologic impairment after stroke and the extent of initial damage is greater in males than females. We have previously shown that spleen cells directly contribute to ischemic damage in males, as splenectomy prior to experimental stroke eliminates the sex differences in infarct volume. This study aims to determine which specific subset of immune cells exert pathogenic effects when injected 24 hours before MCAO induction into splenectomized male and female WT mice. The results demonstrate that CD4/CD8/CD11b treated mice had no significant effect on infarct volumes vs. vehicle-treated control mice after MCAO. However, there were significant alterations to the resident peripheral immune composition. These results suggest that there are regulatory factors resulting from splenectomy or other possible influences that inhibit peripheral immune cell contribution to neuroinflammation and thus contributing to differential effects of the spleen on stroke outcome in males and female mice.

PD-L1 Monoclonal Antibody Treats Ischemic Stroke by Controlling Central Nervous System Inflammation

BACKGROUND AND PURPOSE

Both pathogenic and regulatory immune processes are involved in the middle cerebral artery occlusion (MCAO) model of experimental stroke, including interactions involving the programmed death 1 (PD-1) receptor and its 2 ligands, PD-L1 and PD-L2. Although PD-1 reduced stroke severity, PD-L1 and PD-L2 appeared to play pathogenic roles, suggesting the use of anti-PD-L monoclonal antibody therapy for MCAO.

METHODS

Male C57BL/6 mice were treated with a single dose of anti-PD-L1 monoclonal antibody 4 hours after MCAO and evaluated for clinical, histological and immunologic changes after 96 hours of reperfusion.

RESULTS

Blockade of the PD-L1 checkpoint using a single injection of 200 μg anti-PD-L1 monoclonal antibody given intravenously 4 hours after occlusion significantly reduced MCAO infarct volumes and improved neurological outcomes after 96 hours of reperfusion. Treatment partially reversed splenic atrophy and decreased central nervous system infiltrating immune cells concomitant with enhanced appearance of CD8(+) regulatory T cells in the lesioned central nervous system hemisphere.

CONCLUSIONS

This study demonstrates for the first time the beneficial therapeutic effects of PD-L1 checkpoint blockade on MCAO, thus validating proposed mechanisms obtained in our previous studies using PD-1- and PD-L-deficient mice. These results provide strong support for the use of available humanized anti-PD-L1 antibodies for treatment of human stroke subjects.

Recombinant T-cell receptor ligand RTL1000 limits inflammation and decreases infarct size after experimental ischemic stroke in middle-aged mice

We have previously demonstrated that recombinant T-cell receptor ligand 1000 (RTL1000) reduces infarct size and improves long-term functional recovery after experimental stroke in young transgenic mice expressing human leukocyte antigen DR2 (DR2-Tg). In this study, we determined the effect of RTL1000 on infarct size in 12-month-old middle-aged DR2-Tg mice, and investigated its mechanism of action. Twelve-month-old male DR2-Tg mice underwent 60min of intraluminal reversible middle cerebral artery occlusion (MCAO). Vehicle or RTL1000 was injected 4, 24, 48 and 72h after MCAO. Cortical, striatal and total hemispheric infarcts were measured 96h after stroke. Spleen and brain tissues were collected 96h after stroke for immunological analysis. Our data showed that RTL1000 significantly reduced infarct size 96h after MCAO in middle-aged male DR2-Tg mice. RTL1000 decreased the number of activated monocytes/microglia cells (CD11b(+)CD45(hi)) and CD3(+) T cells in the ischemic hemisphere. RTL1000 also reduced the percentage of total T cells and inflammatory neutrophils in the spleen. These findings suggest that RTL1000 protects against ischemic stroke in middle-aged male mice by limiting post-ischemic inflammation.

Different immunological mechanisms govern protection from experimental stroke in young and older mice with recombinant TCR ligand therapy

Stroke is a leading cause of death and disability in the United States. The lack of clinical success in stroke therapies can be attributed, in part, to inadequate basic research on aging rodents. The current study demonstrates that recombinant TCR ligand therapy uses different immunological mechanisms to protect young and older mice from experimental stroke. In young mice, RTL1000 therapy inhibited splenocyte efflux while reducing frequency of T cells and macrophages in the spleen. Older mice treated with RTL1000 exhibited a significant reduction in inflammatory cells in the brain and inhibition of splenic atrophy. Our data suggest age specific differences in immune response to stroke that allow unique targeting of stroke immunotherapies.

Omalizumab may decrease IgE synthesis by targeting membrane IgE+ human B cells

Background

Omalizumab, is a humanized anti-IgE monoclonal antibody used to treat allergic asthma. Decreased serum IgE levels, lower eosinophil and B cell counts have been noted as a result of treatment. In vitro studies and animal models support the hypothesis that omalizumab inhibits IgE synthesis by B cells and causes elimination of IgE-expressing cells either by induction of apoptosis or induction of anergy or tolerance.

Methods

We examined the influence of omalizumab on human tonsillar B cell survival and on the genes involved in IgE synthesis. Tonsillar B cells were stimulated with IL-4 plus anti-CD40 antibody to induce class switch recombination to IgE production in the presence or absence of omalizumab. Cell viability was assessed and RNA extracted to examine specific genes involved in IgE synthesis.

Conclusions

We found that omalizumab reduced viable cell numbers but this was not through induction of apoptosis. IL-4R and germline Cϵ mRNA levels were decreased as well as the number of membrane IgE+ cells in B cells treated with omalizumab. These data suggest that omalizumab may decrease IgE synthesis by human B cells by specifically targeting membrane IgE-bearing B cells and inducing a state of anergy.

Elimination of T cell reactivity to pancreatic β cells and partial preservation of β cell activity by peptide blockade of LFA-1:ICAM-1 interaction in the NOD mouse model

In insulin dependent diabetes mellitus (T1D), self-reactive T cells infiltrate pancreatic islets and induce beta cell destruction and dysregulation of blood glucose. A goal is to control only the self-reactive T cells, leaving the remainder of the T cell population free to protect the host. One approach is blockade of the second signal for T cell activation while allowing the first (antigen-specific) signal to occur. This work proposes that small peptides that block interaction of second signals delivered through the counter receptors LFA-1:ICAM-1 will induce attacking T cells (receiving the antigen signal) to become anergic or undergo apoptosis. In NOD mice, the peptides eliminated T cell reactivity against pancreatic antigens and reduced cellular infiltration into islets, which retained stronger density of insulin staining at five weeks after cessation of therapy. In in vitro studies the peptides induced nonresponsiveness during activation of T cells from mice and from human peripheral blood.

Variations in rodent models of type 1 diabetes: islet morphology

Type 1 diabetes (T1D) is characterized by hyperglycemia due to lost or damaged islet insulin-producing β -cells. Rodent models of T1D result in hyperglycemia, but with different forms of islet deterioration. This study focused on 1 toxin-induced and 2 autoimmune rodent models of T1D: BioBreeding Diabetes Resistant rats, nonobese diabetic mice, and Dark Agouti rats treated with streptozotocin. Immunochemistry was used to evaluate the insulin levels in the β -cells, cell composition, and insulitis. T1D caused complete or significant loss of β -cells in all animal models, while increasing numbers of α -cells. Lymphocytic infiltration was noted in and around islets early in the progression of autoimmune diabetes. The loss of lymphocytic infiltration coincided with the absence of β -cells. In all models, the remaining α - and δ -cells regrouped by relocating to the islet center. The resulting islets were smaller in size and irregularly shaped. Insulin injections subsequent to induction of toxin-induced diabetes significantly preserved β -cells and islet morphology. Diabetes in animal models is anatomically heterogeneous and involves important changes in numbers and location of the remaining α - and δ -cells. Comparisons with human pancreatic sections from healthy and diabetic donors showed similar morphological changes to the diabetic BBDR rat model.

Choice of resident costimulatory molecule can influence cell fate in human naïve CD4+ T cell differentiation

With antigen stimulation, naïve CD4+ T cells differentiate to several effector or memory cell populations, and cytokines contribute to differentiation outcome. Several proteins on these cells receive costimulatory signals, but a systematic comparison of their differential effects on naïve T cell differentiation has not been conducted. Two costimulatory proteins, CD28 and ICAM-1, resident on human naïve CD4+ T cells were compared for participation in differentiation. Under controlled conditions, and with no added cytokines, costimulation through either CD3+CD28 or CD3+CAM-1 induced differentiation to T effector and T memory cells. In contrast, costimulation through CD3+ICAM-1 induced differentiation to Treg cells whereas costimulation through CD3+CD28 did not.

Brief costimulation through ICAM-1 of human naïve CD4+ T cells induces differentiation to a memory phenotype days later (83.9)

We previously demonstrated that costimulation through ICAM-1 induces differentiation of human naïve CD4+ T cells to a memory phenotype. Similar to results of costimulation through CD28, the phenotype includes expression of a cell surface protein array characteristic of T memory and a robust proliferative and cytokine response to low-level stimulation through the TCR complex alone. In the present work we describe the kinetics of differentiation in response to costimulation through ICAM-1 as compared with CD28 and define the minimal time that costimulatory signals are required for the naive T cells to achieve the memory phenotype. In addition, costimulation through ICAM-1 induces T cells with distinct homing properties.

This work was supported by the National Institute on Aging/NIH.

Costimulation through ICAM-1 induces the differentiation of human naïve CD4+ T cells to regulatory T cells: acquisition of distinct signaling and homing profiles during differentiation (89.39)

Inducible regulatory T (Treg) cells are important mediators of peripheral tolerance that arise by mechanisms not fully defined. We have previously shown that a subset of human naïve CD4+ T cells that are costimulated through Intercellular Adhesion Molecule-1 (ICAM-1) will differentiate into cells with a Foxp3+CD25+CD127(-) phenotype and suppressive function characteristic of inducible Treg cells. Our present work identifies some of the signaling pathways that are activated during this differentiation process. We also characterize some of the chemokine receptors and other homing markers that are displayed on these Treg cells, including CCR7, CD62L, and specific integrins. These results help clarify the signaling requirements for in vitro differentiation of human naïve CD4+ T cells to Treg cells after costimulation through ICAM-1 and suggest possible microenvironments in which these cells could migrate and function during immune responses in vivo.

This work was supported by the National Institute on Aging/NIH.

Costimulation Through ICAM-1 Induces Differentiation of Human Naïve CD4+ T Cells to Regulatory T Cells (88.32)

Inducible regulatory T cells can be generated from precursor cells in the periphery. However, the specific precursors and the mechanism of Treg differentiation are not well characterized. We investigated the participation of Intercellular Adhesion Molecule-1 (ICAM-1) in the process of Treg induction beginning with human naïve CD4+ T cells, defined as CD45RA+CD45RO-CD11aloCD27+CCR7+. Costimulation of naïve CD4+ T cells through ICAM-1 (but not CD28) resulted in the appearance of a sizeable population of Foxp3+CD25+CD4+ Treg cells. This stimulation treatment also generated substantial quantities of the immunosuppressive cytokine IL-10. The Treg cells that differentiated following costimulation through ICAM-1 strongly inhibited responder cell proliferation in co-culture experiments. The ability of naïve CD4+ T cells to undergo this differentiation process was examined using cells from both young individuals and older individuals. These results indicate that ICAM-1 resident on the surface of naïve CD4+ T cells can mediate the differentiation of Tregs from naïve cells and suggest that ICAM-1 may be a signaling molecule to target for Treg therapies.

Costimulation through ICAM-1 induces differentiation of naive human CD4+ T cells from aging individuals (85.27)

With age, the immune response is less effective at protection and less responsive to vaccines. The events leading to immunosenescence have not been fully described. We have been investigating the hypothesis that the nature of the T cell costimulatory signal governs in part the phenotype of the post thymic developing naïve T cell as it responds to cognate antigen. The costimulatory molecules, CD28 and LFA-1 are the best characterized; however we have demonstrated that ICAM-1, resident on a T cell, can receive signals that drive naïve human CD4+ T cells taken from young individuals to effector and memory phenotypes that differ from those induced by CD28 or LFA-1. For the present study, we are investigating the participation of these three costimulatory signals in activation and differentiation of human naive CD4+ T cells from aging individuals. We will describe differential effects on development in response to costimulation of naïve T cells taken from older as compared with young human subjects.