Immunodeficiency impairs re-injury induced reversal of neuronal atrophy: relation to T cell subsets and microglia

CD4+ T cell expression of the IL-10 receptor is necessary for facial motoneuron survival after axotomy

Background

After peripheral nerve transection, facial motoneuron (FMN) survival depends on an intact CD4+ T cell population and a central source of interleukin-10 (IL-10). However, it has not been determined previously whether CD4+ T cells participate in the central neuroprotective IL-10 cascade after facial nerve axotomy (FNA).

Methods

Immunohistochemical labeling of CD4+ T cells, pontine vasculature, and central microglia was used to determine whether CD4+ T cells cross the blood-brain barrier and enter the facial motor nucleus (FMNuc) after FNA. The importance of IL-10 signaling in CD4+ T cells was assessed by performing adoptive transfer of IL-10 receptor beta (IL-10RB)-deficient CD4+ T cells into immunodeficient mice prior to injury. Histology and qPCR were utilized to determine the impact of IL-10RB-deficient T cells on FMN survival and central gene expression after FNA. Flow cytometry was used to determine whether IL-10 signaling in T cells was necessary for their differentiation into neuroprotective subsets.

Results

CD4+ T cells were capable of crossing the blood-brain barrier and associating with reactive microglial nodules in the axotomized FMNuc. Full induction of central IL-10R gene expression after FNA was dependent on CD4+ T cells, regardless of their own IL-10R signaling capability. Surprisingly, CD4+ T cells lacking IL-10RB were incapable of mediating neuroprotection after axotomy and promoted increased central expression of genes associated with microglial activation, antigen presentation, T cell co-stimulation, and complement deposition. There was reduced differentiation of IL-10RB-deficient CD4+ T cells into regulatory CD4+ T cells in vitro.

Conclusions

These findings support the interdependence of IL-10- and CD4+ T cell-mediated mechanisms of neuroprotection after axotomy. CD4+ T cells may potentiate central responsiveness to IL-10, while IL-10 signaling within CD4+ T cells is necessary for their ability to rescue axotomized motoneuron survival. We propose that loss of IL-10 signaling in CD4+ T cells promotes non-neuroprotective autoimmunity after FNA.

Facial Nerve Recovery in KbDb and C1q Knockout Mice: A Role for Histocompatibility Complex 1

BACKGROUND

Understanding the mechanisms in nerve damage can lead to better outcomes for neuronal rehabilitation. The purpose of our study was to assess the effect of major histocompatibility complex I deficiency and inhibition of the classical complement pathway (C1q) on functional recovery and cell survival in the facial motor nucleus (FMN) after crush injury in adult and juvenile mice.

METHODS

A prospective blinded analysis of functional recovery and cell survival in the FMN after a unilateral facial nerve crush injury in juvenile and adult mice was undertaken between wild-type, C1q knockout (C1q-/-), and KbDb knockout (KbDb-/-) groups. Whisker function was quantified to assess functional recovery. Neuron counts were performed to determine neuron survival in the FMN after recovery.

RESULTS

After facial nerve injury, all adult wild-type mice fully recovered. Juvenile mice recovered incompletely corresponding to a greater neuron loss in the FMN of juveniles compared with adults. The C1q-/- juvenile and adult groups did not differ from wild type. The KbDb-/- adults demonstrated 50% recovery of whisker movement and decreased cell survival in FMN. The KbDb-/- juvenile group did not demonstrate any difference from control group.

CONCLUSION

Histocompatibility complex I plays a role for neuroprotection and enhanced facial nerve recovery in adult mice. Inhibition of the classical complement pathway alone does not affect functional recovery or neuronal survival. The alternative and mannose binding pathways pose alternative means for activating the final components of the pathway that may lead to acute nerve damage.

Reversal of Neuronal Atrophy: Role of Cellular Immunity in Neuroplasticity and Aging

Emerging evidence indicates that neuroimmunological changes in the brain can modify intrinsic brain processes that are involved in regulating neuroplasticity. Increasing evidence suggests that in some forms of motor neuron injury, many neurons do not die, but reside in an atrophic state for an extended period of time. In mice, facial motor neurons in the brain undergo a protracted period of degeneration or atrophy following resection of their peripheral axons. Reinjuring the proximal nerve stump of the chronically resected facial nerve stimulates a robust reversal of motor neuron atrophy which results in marked increases in both the number and size of injured motor neurons in the facial motor nucleus. In this brief review, we describe research from our lab which indicates that the reversal of atrophy in this injury model is dependent on normal cellular immunity. The role of T cells in this unique form of neuroplasticity following injury and in brain aging, are discussed. The potential role of yet undiscover intrinsic actions of recombination activating genes in the brain are considered. Further research using the facial nerve reinjury model could identify molecular signals involved in neuroplasticity, and lead to new ways to stimulate neuroregenerative processes in neurotrauma and other forms of brain insult and disease.

5. T cell immunity and neuroplasticity

The proneuronal effects of T cells that impact the brain occur from both T cells trafficking into the brain, and from signals in the periphery (e.g., cytokine release and regulation). Recent data indicates that neuroimmunological changes in the brain can modify intrinsic brain processes that are involved in regulating neuroplasticity (e.g., T-cell/microglial interactions, neurotrophins, neurogenesis). We describe: 1) work from our lab and others showing that injury-induced loss of neuronal phenotype and reversal of motor neuron atrophy are associated with normal T cell immunity, and; 2) research indicating that these and other neuroimmunological processes may be generalizable to mechanisms of neuroplasticity involved in cognitive and emotional behavior. These findings are discussed in relation to our lab's working hypothesis, that T cell immunosenesence may contribute to alterations in brain neuroplasticity related to aging. Greater understanding of the role of adaptive T cell immunity on neuroplasticity could have important clinical implications for developing novel treatment strategies for neurodegenerative diseases (e.g., Alzheimer's) and brain injury (e.g., stroke, trauma).

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).

Dissecting the effects of endogenous brain IL-2 and normal versus autoreactive T lymphocytes on microglial responsiveness and T cell trafficking in response to axonal injury

IL-2 is essential for T-helper regulatory (Treg) cell function and self-tolerance, and dysregulation of both endogenous brain and peripheral IL-2 gene expression may have important implications for neuronal injury and repair. We used an experimental approach combining mouse congenic breeding and immune reconstitution to test the hypothesis that the response of motoneurons to injury is modulated by the combined effects of IL2-mediated processes in the brain that modulate its endogenous neuroimmunological milieu, and IL2-mediated processes in the peripheral immune system that regulate T cell function (i.e., normal versus autoreactive Treg-deficient T cells). This experimental strategy enabled us to test our hypothesis by disentangling the effect of normal versus autoreactive T lymphocytes from the effect of endogenous brain IL-2 on microglial responsiveness (microglial phagocytic clusters normally associated with dead motoneurons and MHC2(+) activated microglia) and T cell trafficking, using the facial nerve axotomy model of injury. The results demonstrate that the loss of both brain and peripheral IL-2 had an additive effect on numbers of microglial phagocytic clusters at day 14 following injury, whereas the autoreactive status of peripheral T cells was the primary factor that determined the degree to which T cells entered the injured brain and contributed to increased microglial phagocytic clusters. Changes in activated MHC2(+) microglial in the injured FMN were associated with loss of endogenous brain IL-2 and/or peripheral IL-2. This model may provide greater understanding of the mechanisms involved in determining if T cells entering the injured central nervous system (CNS) have damaging or proregenerative effects.

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.

Exacerbation of facial motoneuron loss after facial nerve axotomy in CCR3-deficient mice

We have previously demonstrated a neuroprotective mechanism of FMN (facial motoneuron) survival after facial nerve axotomy that is dependent on CD4⁺ Th2 cell interaction with peripheral antigen-presenting cells, as well as CNS (central nervous system)-resident microglia. PACAP (pituitary adenylate cyclase-activating polypeptide) is expressed by injured FMN and increases Th2-associated chemokine expression in cultured murine microglia. Collectively, these results suggest a model involving CD4⁺ Th2 cell migration to the facial motor nucleus after injury via microglial expression of Th2-associated chemokines. However, to respond to Th2-associated chemokines, Th2 cells must express the appropriate Th2-associated chemokine receptors. In the present study, we tested the hypothesis that Th2-associated chemokine receptors increase in the facial motor nucleus after facial nerve axotomy at timepoints consistent with significant T-cell infiltration. Microarray analysis of Th2-associated chemokine receptors was followed up with real-time PCR for CCR3, which indicated that facial nerve injury increases CCR3 mRNA levels in mouse facial motor nucleus. Unexpectedly, quantitative- and co-immunofluorescence revealed increased CCR3 expression localizing to FMN in the facial motor nucleus after facial nerve axotomy. Compared with WT (wild-type), a significant decrease in FMN survival 4 weeks after axotomy was observed in CCR3^−/− mice. Additionally, compared with WT, a significant decrease in FMN survival 4 weeks after axotomy was observed in Rag2^−/− (recombination activating gene-2-deficient) mice adoptively transferred CD4⁺ T-cells isolated from CCR3^−/− mice, but not in CCR3^−/− mice adoptively transferred CD4⁺ T-cells derived from WT mice. These results provide a basis for further investigation into the co-operation between CD4⁺ T-cell- and CCR3-mediated neuroprotection after FMN injury.

Effects of facial nerve axotomy on Th2- and Th1-associated chemokine expression in the facial motor nucleus of wild-type and presymptomatic mSOD1 mice

We have previously demonstrated a neuroprotective mechanism of facial motoneuron (FMN) survival after facial nerve axotomy that is dependent on CD4(+) Th2 cell interaction with peripheral antigen-presenting cells, as well as CNS resident microglia. To investigate this mechanism, we chose to study the Th2-associated chemokine, CCL11, and Th1-associated chemokine, CXCL11, in wild-type and presymptomatic mSOD1 mice after facial nerve axotomy. In this report, the results indicate that CCL11 is constitutively expressed in the uninjured facial motor nucleus, but CXCL11 is not expressed at all. Facial nerve axotomy induced a shift in CCL11 expression from FMN to astrocytes, whereas CXCL11 was induced in FMN. Differences in the number of CCL11- and CXCL11-expressing cells were observed between WT and mSOD1 mice after facial nerve axotomy.

T cell memory in the injured facial motor nucleus: relation to functional recovery following facial nerve crush

T cells have the ability to mount a memory response to a previously encountered antigen such that re-exposure to the antigen results in a response that is greater in magnitude and function. Following facial nerve transection, T cells have been shown to traffic to injured motor neurons in the facial motor nucleus (FMN) and may have the ability to promote neuronal survival and functional recovery. Previously, we demonstrated that early exposure to neuronal injury on one side of the brain during young adulthood elicited a T cell response that was greater in magnitude following exposure to the same form of injury on the contralateral side later in adulthood. Whether the T cell memory response to neuronal injury influenced functional recovery following nerve crush injury was unknown. In the current study, we tested the hypotheses that (1) transection of the right facial nerve in sensitized mice would result in faster recovery of the whisker response when the contralateral facial nerve is crushed 10 weeks later, and (2) the early recovery would be associated with an increase in the magnitude of the T cell response in the contralateral FMN following crush injury in sensitized mice. The onset of modest recovery in sensitized mice occurred between 3 and 5 days following crush injury of the contralateral facial nerve, approximately 1.5 days earlier than naïve mice, and was associated with more than a two-fold increase in the magnitude of the T cell response in the contralateral FMN following crush injury. There was no difference between groups in the number of days to full recovery. Further study of how T cell memory influences neuroregeneration may have important implications for translational research.

Influence of injury severity on the rate and magnitude of the T lymphocyte and neuronal response to facial nerve axotomy

The temporal relationship between severity of peripheral axonal injury and T lymphocyte trafficking to the neuronal cell bodies of origin in the brain has been unclear. We sought to test the hypothesis that greater neuronal death induced by disparate forms of peripheral nerve injury would result in differential patterns of T cell infiltration and duration at the cell bodies of origin in the brain and that these measures would correlate with the magnitude of neuronal death over time and cumulative neuronal loss. To test this hypothesis, we compared the time course of CD3(+) T cell infiltration and neuronal death (assessed by CD11b(+) perineuronal microglial phagocytic clusters) following axonal crush versus axonal resection injuries, two extreme variations of facial nerve axotomy that result in mild versus severe neuronal loss, respectively, in the facial motor nucleus. We also quantified the number of facial motor neurons present at 49 days post-injury to determine whether differences in the levels of neuronal death between nerve crush and resection correlated with differences in cumulative neuronal loss. Between 1 and 7 days post-injury when levels of neuronal death were minimal, we found that the rate of accumulation and magnitude of the T cell response was similar following nerve crush and resection. Differences in the T cell response were apparent by 14 days post-injury when the level of neuronal death following resection was substantially greater than that seen in crush injury. For nerve resection, the peak of neuronal death at 14 days post-resection was followed by a maximal T cell response one week later at 21 days. Differences in the level of neuronal death between the two injuries across the time course tested reflected differences in cumulative neuronal loss at 49 days post-injury. Altogether, these data suggest that the trafficking of T cells to the injured FMN is dependent upon the severity of peripheral nerve injury and associated neuronal death.