Interleukin-6 (IL6) and cellular response to facial nerve injury: effects on lymphocyte recruitment, early microglial activation and axonal outgrowth in IL6-de®cient mice Matthias Galiano, 1 Zhi Qiang Liu, 1 Roger Kalla, 1 Marion Bohatschek, 1 Andrea Koppius, 1 Andreas Gschwendtner, 1 ShengLi Xu, 1 Alexander Werner, 1 Christian U.A. Kloss, 1 Leonard L. Jones, 1 Horst Bluethmann 2 and Gennadij Raivich 1,3 1 Department of Neuromorphology, Max-Planck Institute for Neurobiology, Am Klopferspitz 18A, D-82152 Martinsried, Germany 2 Hoffmann-LaRoche, Basle, Switzerland 3 Perinatal Brain Repair Group, Department of Obstetrics and Gynaecology and Department of Anatomy, University College London, 86±96 Chenies Mews, London WC1E 6HX, UK Keywords: granulocyte, macrophage, nerve regeneration, neuronal cell death, T-cell Abstract Nerve injury triggers numerous changes in the injured neurons and surrounding non-neuronal cells. Of particular interest are molecular signals that play a role in the overall orchestration of this multifaceted cellular response. Here we investigated the function of interleukin-6 (IL6), a multifunctional neurotrophin and cytokine rapidly expressed in the injured nervous system, using the facial axotomy model in IL6-de®cient mice and wild-type controls. Transgenic deletion of IL6 caused a massive decrease in the recruitment of CD3-positive T-lymphocytes and early microglial activation during the ®rst 4 days after injury in the axotomized facial nucleus. This was accompanied by a more moderate reduction in peripheral regeneration at day 4, lymphocyte recruitment (day 14) and enhanced perikaryal sprouting (day 14). Motoneuron cell death, phagocytosis by microglial cells and recruitment of granulocytes and macrophages into injured peripheral nerve were not affected. In summary, IL6 lead to a variety of effects on the cellular response to neural trauma. However, the particularly strong actions on lymphocytes and microglia suggest that this cytokine plays a central role in the initiation of immune surveillance in the injured central nervous system. Introduction Peripheral nerve injury initiates a large number of morphological and molecular changes in the damaged nervous system that are particu- larly pronounced at three different sites: at the site of axotomy, in the cell body of injured neurons, and in the surrounding non-neuronal cells. In the injured peripheral nerve, the disconnected nerve ®bres and associated myelin distal to the lesion site undergo Wallerian degeneration, a process assisted by the in¯ux of haematogenous leucocytes, particularly the macrophages. In the proximal part, the tips of the cut axons transform into growth cones of sprouting axons; these reenter endoneural tubes in the distal nerve, grow back and reinnervate peripheral targets. The initially naked axonal shafts are also gradually remyelinated by the surrounding Schwann cells, restoring their normal physiological function. At the neuronal cell body, the injured neurons display morpho- logical and metabolic changes, characterized as chromatolysis, retrograde reaction or cell body response, reviewed in detail by Lieberman (1971), Grafstein & McQuarrie (1978) and Raivich et al., 1999a). On a molecular level, injured neurons show an increase in transcription factors (Herdegen et al., 1991, 1998; Haas et al., 1993; Yao et al., 1997; Schwaiger et al., 2000), growth-associated proteins (Benowitz et al., 1981; Willard & Skene, 1982), neuropeptides (Moore, 1989; Raivich et al., 1995), cytokines and neurotrophins (Klein et al., 1997; Gschwendtner et al., 1999; Murphy et al., 1999a; Streit et al., 2000) and cell adhesion molecules (Mo Èller et al., 1996; Jones et al., 1997, 2000). These molecular changes, in particular the induction of cell adhesion molecules such as the laminin receptor a7b1 integrin, serve as an integral part of the neuronal regeneration program (Werner et al., 2000). In addition to the periphery and the neuronal perikaryon, axonal injury will also lead to the activation of non-neuronal cells around the cell bodies of axotomized neurons. In the CNS, the microglial cells increase in adhesion molecules (Mo Èller et al., 1996; Werner et al., 1998; Kloss et al., 1999) and home onto and adhere to damaged neurons (Blinzinger & Kreutzberg, 1968; Kalla et al., 2000). They express major compatibility complex glycoproteins and costimulatory factors needed for antigen presentation (Streit et al., 1989; Bohatschek et al., 1999) and interact with T-lymphocytes recruited to the injured brain (Raivich et al., 1998a; Werner et al., 1998; Jones et al., 2000). Adjacent blood vessels show an increase in cell adhesion molecules which may assist leucocyte recruitment (Kloss et al., 1999). The neighbouring astrocytes show an induction of cytoskeletal molecules such as GFAP and vimentin (Bignami et al., 1974; Graeber & Kreutzberg, 1986; 1988), rearrange their cytoskeleton into a stellar shape (Raivich et al., 1999a,b) and increase the physical stability of the damaged neural parenchyma (Pekny et al., 1999). At present, the actual contribution of this central non-neuronal response to neural repair is surrounded by a high degree of Correspondence: Dr Gennadij Raivich, 3 Perinatal Brain Repair Group, as above. E-mail: g.raivich@ucl.ac.uk or raivich@neuro.mpg.de Received 22 January 2001, revised 15 May 2001, accepted 29 May 2001 European Journal of Neuroscience, Vol. 14, pp. 327±341, 2001 ã Federation of European Neuroscience Societies