Modulating MAC in Traumatic Brain Injury
The complement system has been identified as an early mediator of neuroinflammation and neuropathology following traumatic brain injury (TBI). Therapies to improve neurologic outcomes in TBI patients have focused on broad inhibition of the complement cascade, which can limit effective immune responses against infections in TBI patients. To evaluate the role of the membrane attack complex (MAC) of the complement system in TBI, Fluiter et al. (p. 2339) used a mouse model of mild and severe closed head injury. Immunostaining of injured brain tissue with C9, a marker of MAC deposition, and markers of activated microglia/macrophages demonstrated that severe TBI resulted in increases in MAC deposition and inflammation relative to both mild TBI and uninjured mice. To block MAC formation without altering early complement activation, the authors designed antisense oligonucleotides targeting C6 production and compared its therapeutic efficacy to the Ornithodoros moubata complement inhibitor (OmCI), a known inhibitor of C5 activation. Pretreatment of TBI mice with C6 antisense or treatment with OmCI up to 15 min post-TBI reduced weight loss, neuroaxonal apoptosis and loss, MAC deposition, and neuroinflammation in the brain and improved neurologic performance at 72 h postinjury. This study demonstrates that MAC inhibition can improve neurologic recovery after TBI and proposes a novel therapeutic target for the treatment of TBI.
Supercharged Neutrophil Responses
The “genomic storm” following traumatic injury is characterized by a modulation of both innate and adaptive immunity genes, leading to suppression of adaptive immunity, enhancement of innate immunity, and significant changes in hematopoiesis, marked by an expansion of myeloid cells. Although traumatic injury is thought to have an overall suppressive effect on the immune system, Gardner et al. (p. 2405) demonstrated that thermal injury conferred protection against lethal Klebsiella pneumoniae challenge. Burn-injured mice (15% total body surface burn) had a marked survival advantage over sham-injured mice when challenged with K. pneumoniae 7 d postinjury. The resistance to infection in burn-injured mice was accompanied by a significant neutrophilic response in the lungs and blood, and protection could be completely reversed by treatment with a neutrophil-depleting Ab. A significant shift toward CD11b+ myeloid lineage cells in the bone marrow was accompanied by an increase in serum G-CSF and IL-6, concomitant with activation of the STAT3 pathway in inoculated burn-injured compared with sham-injured mice. Neutralization of G-CSF blocked 80% of the thermal-injury-induced STAT3 activation in bone marrow progenitor cells, thereby preventing neutrophilia. Conversely, administration of recombinant G-CSF increased neutrophil numbers and conferred a survival advantage in K. pneumoniae-inoculated mice. Taken together, these data suggest a key role for the G-CSF/STAT3 axis in neutrophil responses against infection following traumatic injury.
E2A Fights Allelic Exclusion
It is known that a negative feedback loop initiated by pre-BCR activation and Ca2+ signaling is required for allelic exclusion to occur during V(D)J recombination of Igh genes, but the details of the allelic exclusion molecular mechanism remain an enigma. Hauser et al. (p. 2460) set out to determine if regulation of E2A, a transcription factor known to be expressed during V(D)J recombination, by the Ca2+-sensor protein calmodulin plays a role in this process. To do this, the authors adoptively transferred wild-type (WT) or calmodulin-resistant E2A-expressing bone marrow cells from E2A+/− C57BL/6 X BALB/c F1 mice into lethally-irradiated recipients capable of expressing IgM allotypes from either parent strain. They found that the propensity of B cells from these mice to express both the BALB/c and C57BL/6 allotypes of IgM on the same cell statistically increased in calmodulin-resistant E2A-expressing B cells, suggesting that these cells are defective in allelic exclusion. Pre-BCR signaling-initiated inhibition of transcription factors involved in V(D)J recombination was reduced in calmodulin-resistant E2A-expressing B cells, indicating that regulation of these molecules is dependent on E2A sensitivity to calmodulin. Data from chromatin immunoprecipitation analyses suggested that the calmodulin-resistant E2A sequestered the recombination complex on the Igh locus, whereas WT E2A allowed this complex to be released. Together, these results suggest that calmodulin regulation of E2A plays an integral role in the allelic exclusion process.
IgA Helps NET Bacteria
The expulsion of neutrophil extracellular traps (NETs), which trap and kill pathogens, leads to neutrophil cell death, referred to as NETosis. Previous work by Aleyd et al. (p. 2374) demonstrated that the IgA Fc receptor (FcαRI) triggered the activation of polymorphonuclear cells (PMNs), such as neutrophils, leading to enhanced phagocytosis of IgA-opsonized bacteria and PMN trafficking. In this study, the authors investigated whether FcαRI promotes NET formation. Stimulation of PMNs with Staphylococcus aureus in the presence of serum IgA led to an expansion in PMN size and degradation which increased over time relative to unstimulated PMNs or PMNs stimulated in the absence of IgA. Real time video recordings of unstimulated and S. aureus-stimulated PMNs labeled with Organelle ID, which fluorescently labels organelles with different colors, demonstrated hallmarks of apoptosis, such as Annexin-V staining followed by expression the cell death marker 7-AAD. In contrast, PMNs incubated with IgA-opsonized S. aureus demonstrated diffusion of nuclear contents throughout the cell, cell membrane disintegration, the expression of 7-AAD without Annexin-V, and the formation of NETs. Incubation of PMNs with IgA-coated beads, but not control BSA-coated beads, increased PMN phagocytosis and NET induction which could be abrogated by blocking either FcαRI or ROS production. The stimulation of phagocytosis and NETosis by IgA suggests a mechanism by which IgA plays a prominent role in mucosal inflammatory responses.
Binding Potential TAPped
TAP is a heterodimer composed of two protein components, TAP1 and TAP2, and has essential functions in the MHC I Ag processing pathway, one of which is to interact with the chaperone protein tapasin to facilitate loading of peptide Ag onto MHC I molecules. Tapasin promotes TAP stability and has three known binding sites on TAP1, two in the N-terminus and one in the core transmembrane domain (coreTMD), but it is unclear which of these three sites are required for tapasin to perform this function. The coreTMD tapasin–binding site on TAP1 is poorly characterized, prompting Leonhardt et al. (p. 2480) to investigate where and how tapasin binds the coreTMD of TAP1 and whether this binding site is involved in stabilization of the TAP heterodimer. To do this, the authors performed mutagenesis of the six segments (TM5-TM10) of the coreTMD of TAP1 and stably expressed the mutants in human lymphoblastoid T2 cells. Immunoprecipitation of these mutants with an anti-TAP1 Ab revealed TM9 as the tapasin-binding domain in the coreTMD of TAP1. Further mutagenesis of TM9 identified six integral amino acids on the polar face required for tapasin binding. Mutating key residues in both the N-terminus and coreTMD of TAP1 determined that tapasin docking on both of these domains may be required to achieve full stability of TAP1 and subsequent stable heterodimer formation with TAP2. Together, these data suggest that all three tapasin-binding sites may cooperate to facilitate high transporter protein stability and efficient TAP heterodimerization.
- Copyright © 2014 by The American Association of Immunologists, Inc.
