A Remedy for Lupus through Restraining Rab7 See article p.3792

NRP-1 Says No to DC Maturation See article p.3927

How HIV Hijacks the T Cell Machine See article p.4042

The small GTPase Rab7 is expressed in activated B cells and promotes class switch DNA recombination (CSR) through induction of activation-induced cytidine deaminase (AID). AID is upregulated in the B cells of patients and mice with lupus and mediates CSR that results in the production of large quantities of pathogenic IgG autoantibodies. The involvement of Rab7 in CSR led Lam et al. (p. 3792) to assess whether a specific inhibitor of Rab7, CID 1067700, could target both B cells and plasma cells to suppress the production of autoantibodies in lupus. Rab7 was found to be upregulated, relative to cells from healthy controls, in B cells from mice and patients with lupus, and treatment of activated human or mouse B cells with CID 1067700 inhibited canonical NF-κB activation and AID expression, resulting in impairment of CSR without affecting B cell proliferation or germline transcription of Ig genes. In mouse models of lupus, treatment with CID 1067700 significantly reduced disease manifestations and enhanced survival, and disease amelioration was accompanied by clear reductions in IgG autoantibody but not nonpathogenic IgM responses. As suggested by the reduction in IgG production, CID 1067700 treatment of lupus-prone mice reduced CSR, but no effects were seen on B cell proliferation or on the expression of genes required for plasma cell differentiation or function. In addition, CID 1067700 had no major effects on macrophages, dendritic cells, or T cells in these mice. Inhibition of Rab7 reduced the total numbers of Ab-secreting cells in both lupus-prone and wild-type mice, and this reduction was traced to impairment of plasma cell survival. In support of this mechanism, treatment of plasma cells with CID 1067700 in vitro induced apoptosis, which could be prevented by forced expression of NF-κB. Thus, Rab7 inhibition via CID 1067700 impairs both CSR in B cells and survival of plasma cells. This study encourages the investigation of Rab7 targeting for the treatment of patients with lupus and suggests exploring Rab7 involvement in other diseases involving autoantibodies, as well as in B cell malignancies.

Dendritic cells (DCs) are potent APCs that prime anti-tumor and anti-microbial adaptive immune responses by presenting Ag and providing essential signals directing T cell activation and differentiation. Given the importance of DCs in initiating and polarizing immune responses, understanding how pro- and anti-inflammatory signals from the tissue environment influence DC function is essential. Vascular endothelial growth factor (VEGF), which is abundant in tumors, has been demonstrated to potently inhibit DC maturation and function, potentially by binding neuropilin-1 (NRP-1) on the DC surface. While it is known that DC NRP-1 is involved in the formation of immune synapses with T cells, Oussa et al. (p. 3927) elucidated other functions of NRP-1 by investigating whether the VEGF-1–NRP-1 interaction plays a role in DC maturation and function. Compared with expression levels on DCs to which no VEGF had been added, addition of VEGF to bone marrow–derived DCs (BMDCs) significantly reduced LPS-driven, but not CpG- or polyinosinic:polycytidylic acid-driven, expression of DC maturation markers CD40, CD86, and MHC class II. This phenotypic change correlated with functional impairment, as VEGF-1 reduced BMDC production of inflammatory cytokines IL-1β, IL-6, IL-12, and TNF-α in LPS-matured BMDCs. Using the Cre-Lox system, an NRP-1 conditional knockout mouse strain was generated in which NRP-1 expression in myeloid cells was reduced by 70%. Compared with wild-type controls, conditional deletion of NRP-1 had no impact on the number, phenotype, or function of LPS-matured BMDCs. Addition of VEGF-1 to NRP-1–deficient BMDCs did not alter expression of BMDC maturation markers or inflammatory cytokine secretion when compared with WT controls, suggesting that VEGF requires NRP-1 to inhibit LPS-dependent maturation. Additional studies revealed that, in the presence of VEGF, NRP-1 directly interacted with TLR4 and suppressed downstream signaling through ERK1-2 and NF-кB in BMDCs. Taken together, these data demonstrate that NRP-1 is essential for VEGF inhibition of LPS-driven maturation of BMDCs via inhibition of TLR4 signaling. Furthermore, this study may be of particular relevance to tumor immunology, in which VEGF is suspected to contribute to DC dysfunction and neoplastic progression.

The HIV accessory protein Nef promotes HIV-1 replication, which is itself modulated by T cell activation. Nef may affect the functions of over 180 proteins in T cells, but there is as yet no cohesive understanding of how Nef affects T cell activation and/or differentiation following HIV-1 infection. In this issue, Silva et al. (p. 4042) employed a variety of approaches to better understand how Nef might affect activity at the immunological synapse. During T cell activation, the kinase Lck and the adaptor protein LAT are sequentially delivered via vesicular traffic to the synapse, where they regulate T cell activation by way of an amplification loop involving vesicular fusion. Nef was found to cause vesicular accumulation of Lck through a mechanism requiring the Lck N-terminal domain and association with the Rab11b and MAL GTPases that regulate Lck vesicular trafficking. Nef neither directly affected LAT trafficking nor interacted with the GTPases that reside in LAT-containing vesicles; however, its modulation of Lck trafficking inhibited the positive feedback loop that drives the generation of phosphorylated LAT nanoclusters at the synapse and subsequent formation of phosphorylated LAT-SLP76 signaling complexes that are key to T cell activation. Using a genetically-encoded, membrane-tethered Ca2+ sensor, the authors determined that Nef interfered with the formation of Ca2+ territories in the synaptic membrane, although it did not affect intracellular Ca2+ flux. Restoration of these Ca2+ territories in activated T cells restored LAT nanocluster formation, indicating that Nef modulation of Ca2+ territory formation was responsible for its downstream effects on T cell signaling. Thus, HIV-1 Nef exploits the coordination of vesicle trafficking, signalosome assembly, and localization of Ca2+ signaling to disrupt positive feedback signals that augment TCR signaling, while leaving intracellular Ca2+ signaling intact. This insight into HIV-1 modulation of T cell activation and function may help define potential targets for future antiviral therapies.