Editorial: White blood cells matter in neonatal white-matter injury By Xiaoyang Wang* and Carina Mallard*,†,1 *Department of Neuroscience and Physiology, Sahlgrenska Academy at University of Gothenburg, Sweden; and †Centre for the Developing Brain, Department of Perinatal Imaging and Health, King’s College London, United Kingdom RECEIVED JUNE 5, 2015; REVISED JULY 6, 2015; ACCEPTED JULY 12, 2015. DOI: 10.1189/jlb.3CE0615-242R
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eonatal brain injury remains a common occurrence in preterm infants, and with the enhanced survival of extremely preterm infants, the rates of neonatal brain injury are likely to increase in the future. The most common neuropathological ﬁnding in preterm infants is diffuse injury to the cerebral white matter. The etiology of preterm birth and white-matter injury is often unclear; however, placental insufﬁciency/ fetal hypoxia and infection/inﬂammation are important risk factors. Fourteen years ago, it was ﬁrst suggested that white blood cells might play a signiﬁcant role in neonatal whiter-matter injury . However, it was not until more recently that several studies have been performed to support further the link between peripheral immune cells and neonatal whitematter injury.
Introduction In this issue of Journal of Leukocyte Biology, Ortega and colleagues  used a mouse model of chronic hypoxia, similar to the model established in rats , to investigate further the importance of immune reactions in neonatal brain injury. They demonstrate that a period of chronic hypoxia during the early neonatal period in mice induces a persistent loss of myelin, weeks after the end of the hypoxic exposure. The reduction in myelin correlates with long-term motor impairment, and the authors suggest that the myelin deﬁcit arises from maturational dysfunction of oligodendrocyte Abbreviations: MBP = myelin basic protein
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marker O1-positive cells rather than reduced proliferation capacity of these cells. These results agree with the current understanding of the development of white-matter injury in preterm infants, where arrest of preoligodendrocytes (i.e., O1-positive cells) maturation is believed to be an important mechanism that gives rise to myelination failure . Intriguingly, the authors found increased numbers of CD4+ T cells in the brain, indicating inﬁltration of peripheral adaptive immune cells. When CD4+ T cells were isolated from splenocytes, they were found to be autoreactive to the myelin antigen MBP, which suggests a dynamic interplay between the CNS and the adaptive immune system during the progression of neonatal white-matter injury. Neonates used to be considered to be immune deﬁcient, but it is now widely accepted that even preterm infants can indeed raise an immune response. With the discovery of different T cell populations, it has become evident that immune responses in the newborn are biased toward a Th2/regulatory T cell reaction. However, the adaptive immune system is at a relatively early developmental stage compared with the innate immune system at this age. Thus, in neonates, it appears that there is signiﬁcant contribution of the innate and adaptive immune systems in response to infection/injury , including neonatal brain injury [6, 7] (Fig. 1). However, despite its relative immaturity, the adaptive immune arm seems to play a signiﬁcant role, as blockage of lymphocyte trafﬁcking by use of FTY720 (ﬁngolimod), a sphingosine-1-phosphate receptor agonist,
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prevents inﬂammation-sensitized hypoxicischemic brain injury in newborn rats . A recent study observed peripheral activation of brain antigen-recognizing T cells, several months after acute hypoxiaischemia-induced neonatal brain injury, which might favor the notion that the brain injury in the chronic stage, at least partly, causes inﬁltration of self-antigenreactive T cells . Such autoreactive T cells are very likely playing a different role than autoreactive T cells in multiple sclerosis cases, where these T cells dominate the multiple sclerosis pathology. In the current study, the autoreactivity is speciﬁc to the MBP myelin antigen and not to the other 2 common myelin proteins (myelin oligodendrocyte glycoprotein and proteolipid protein ). All of these myelin peptides appear at similar developmental stages in the brain, so the autoreactive response is unlikely a result of differences in their expression in the white matter. One possibility is that the chronic hypoxia model causes consistent stimulation and/or release speciﬁcally of MBP, which triggers an autoimmune response in splenocytes. However, these ﬁndings need to be conﬁrmed further by animal studies that use different experimental models and by studying human autoimmune antigens in newborn infants with white-matter injury. Important questions that remain to be answered concern the exact relationship between white-matter injury and the adaptive immune response. It remains to be demonstrated whether the MBP 1. Correspondence: Sahlgrenska Academy, University of Gothenburg, Box 432, Gothenburg, Sweden. E-mail: [email protected]
EDITORIAL Wang and Mallard White-matter injury
Figure 1. Illustration of inﬂammatory events that occur after the induction of perinatal white-matter injury. Hypoxia and/or inﬂammation trigger early inﬂammatory responses in the brain, including reactivity of brain-resident microglia and inﬁltration of innate immune cells, such as monocytes, macrophages, neutrophils, and gd T cells that further amplify the inﬂammation in the brain. At the middle stage, activation of resident microglia continues, and there is also inﬁltration of adaptive immune cells, including CD4+ T cells, and myelination deﬁcit starts to appear. At the chronic stage, myelination failure is established in association with astrogliosis. This stage can last for months to years after the initial insults. At each of these stages, it is likely that peripheral immune responses, including both the innate and adaptive immune system, can interact with and affect the inﬂammatory processes in the brain and thus, contribute to the brain injury.
autoreactive CD4+ T cells in the brain are just a side effect of the brain injury or if they are the initiator of white-matter injury. For example, would adoptive transfer of autoreactive T cells from perinatal chronic hypoxia mice to control mice cause brain injury? How important is the contribution of self-reactive CD4+ T cells to the maturation arrest of the preoligodendrocytes? What is the route of CD4+ T cell entry into the neonatal brain? Is the blood-brain barrier permeability increased by hypoxia, thus allowing transfer of cells into the CNS? In light of the new discovery of lymphatic vessels in the CNS , do such lymphocytic vessels also exist in the neonatal brain? If so, how much does the lymphatic system contribute to neonatal brain injury? How translational are these results to human newborns, especially preterm infants? The ontogeny of the adaptive immune system is different in mice compared with humans . For example, we know that mature a/b T cells only increase in number after birth in mice, whereas the ﬁrst mature a/b T cells are present already during the 1st trimester in humans. Another important question that needs to be asked is what role neonatal immunity, inﬂammation, and cytokines play in the normal physiologic development of the CNS—and especially, in the case of preterm birth—in the early developmental stages. Immune responses, both locally in the brain as well as in peripheral tissues, are known to be important for many aspects of brain development,
including neural cell differentiation, cell migration, synapse formation and pruning, and programmed cell death. In relation to this, we should keep in mind that in the perinatal period, both the immune system and the brain are still developing, and when an injury occurs in the brain, this injury can have a signiﬁcant impact on further development of the CNS. If there is indeed an interaction between the immune system and CNS development, it is important to understand how brain injury inﬂuences the developmental processes of both of these systems. The injury might trigger brain cell development and differentiation toward pluripotency, depending on the type of injury that occurs. A deeper understanding of the interplay between neonatal immunity and the CNS and between innate immunity and adaptive immunity under normal and injured conditions will certainly deepen our understanding of the pathologic conditions that occur in newborn infants. There are obviously still many more unknowns regarding the role of immune cells in the development of neonatal white-matter injury, and there are many aspects of this proposed relationship that still need to be investigated. However, based on current knowledge in the ﬁeld and the data from the study by Ortega and colleagues , it is becoming evident that in neonatal white-matter injury, white cells do matter; the question now is in what way they matter. A deeper
understanding of this relationship will pave the way toward the development of effective neuroprotective interventions in newborns.
1. Dammann, O., Durum, S., Leviton, A. (2001) Do white cells matter in white matter damage? Trends Neurosci. 24, 320–324. 2. Ortega, S. B., Kong, X., Venkataraman, R., Kernie, S. G., Stowe, A. M., Raman, L. (2016) Perinatal chronic hypoxia induces hypomyelination and autoreactive myelinspeciﬁc T-cells in the central nervous system. J. Leukoc. Biol. 99, 21–29 3. Ment, L. R., Schwartz, M., Makuch, R. W., Stewart, W. B. (1998) Association of chronic sublethal hypoxia with ventriculomegaly in the developing rat brain. Brain Res. Dev. Brain Res. 111, 197–203. 4. Buser, J. R., Maire, J., Riddle, A., Gong, X., Nguyen, T., Nelson, K., Luo, N. L., Ren, J., Struve, J., Sherman, L. S., Miller, S. P., Chau, V., Hendson, G., Ballabh, P., Grafe, M. R., Back, S. A. (2012) Arrested preoligodendrocyte maturation contributes to myelination failure in premature infants. Ann. Neurol. 71, 93–109. 5. PrabhuDas, M., Adkins, B., Gans, H., King, C., Levy, O., Ramilo, O., Siegrist, C. A. (2011) Challenges in infant immunity: implications for responses to infection and vaccines. Nat. Immunol. 12, 189–194. 6. Hagberg, H., Mallard, C., Ferriero, D. M., Vannucci, S. J., Levison, S. W., Vexler, Z. S., Gressens, P. (2015) The role of inﬂammation in perinatal brain injury. Nat. Rev. Neurol. 11, 192–208. 7. Albertsson, A. M., Bi, D., Duan, L., Zhang, X., Leavenworth, J. W., Qiao, L., Zhu, C., Cardell, S., Cantor, H., Hagberg, H., Mallard, C., Wang, X. (2014) The immune response after hypoxia-ischemia in a mouse model of preterm brain injury. J. Neuroinﬂammation 11, 153. 8. Yang, D., Sun, Y. Y., Bhaumik, S. K., Li, Y., Baumann, J. M., Lin, X., Zhang, Y., Lin, S. H., Dunn, R. S., Liu, C. Y., Shie, F. S., Lee, Y. H.,
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Wills-Karp, M., Chougnet, C. A., Kallapur, S. G., Lewkowich, I. P., Lindquist, D. M., Murali-Krishna, K., Kuan, C. Y. (2014) Blocking lymphocyte trafﬁcking with FTY720 prevents inﬂammation-sensitized hypoxicischemic brain injury in newborns. J. Neurosci. 34, 16467–16481. 9. Winerdal, M., Winerdal, M. E., Kinn, J., Urmaliya, V., Winqvist, O., Ad´en, U. (2012)
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Long lasting local and systemic inﬂammation after cerebral hypoxic ischemia in newborn mice. PLoS One 7, e36422. 10. Louveau, A., Smirnov, I., Keyes, T. J., Eccles, J. D., Rouhani, S. J., Peske, J. D., Derecki, N. C., Castle, D., Mandell, J. W., Lee, K. S., Harris, T. H., Kipnis, J. (2015) Structural and functional features of
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central nervous system lymphatic vessels. Nature 523, 337–341. 11. Dowling, D. J., Levy, O. (2014) Ontogeny of early life immunity. Trends Immunol. 35, 299–310. KEY WORDS: CNS CD4+ T cells neonatal brain injury •