Despite the potential for a broad TCR repertoire generated by V(D)J recombination and junctional diversity, many T cell responses are characterized by biased usage of “public” TCRs among individuals with shared MHC genotypes. Interestingly, insertion/deletion-related polymorphisms have been identified in the TRB (TCR β) locus of individuals from different ethnic groups, which reduce the total number of TRBV gene segments. Brennan et al. (p. 2742) have assessed the impact of a particular TRBV insertion/deletion on CD8+ T cell responses to an 11-aa peptide, called RPH, from the human CMV protein pp65. Consistent with observations from a previous study, CMV-seropositive individuals expressing HLA-B*0702 showed skewed usage of a public TCR β-chain expressed from the TRBV4-3 gene segment in RPH-specific CD8+ T cells. A subset of individuals generated RPH-specific CD8+ T cells via “private” TCR usage using a range of TRBV genes instead of TRBV4-3. Further analysis revealed that these individuals were all homozygous for a 21.5 kB deletion in a region that included TRBV4-3. Crystal structure studies revealed that binding of public and private TCRs to the RPH–HLA-B*0702 complex were differently affected by single alanine substitutions within the RPH peptide. Overall, these results clarify how a polymorphism within the TRB locus can impact the diversity of Ag-specific CD8+ T cell responses to viral Ag.

Several mechanisms have been identified that lead to the development of memory-like T cells in the absence of Ag stimulation, including the emergence of “virtual memory” (VM) CD8+ T cells from thymocytes exposed to IL-4. Akue et al. (p. 2516) have gained insights into the development and maintenance of VM CD8+ T cells in studies using wild-type (WT) and IL-4R−/− mice. VM cells (CD8+CD44hi) were detected in unimmunized mice among precursor pools of T cells with different foreign Ag specificities and did not appear to be dual reactive T cells resulting from incomplete TCR α-chain allelic exclusion. Compared with WT mice, the frequencies of VM cells in IL-4R−/− mice were significantly reduced in two different precursor pools in the periphery and among thymic single positive CD8+ T cells. Analysis of VM cells in neonatal and adult mice indicated that VM cells most likely emerge in the periphery during neonatal lymphopenia and persist through adulthood. In addition, the VM cell population can be maintained long term through basal proliferation, and the frequency of VM cells was not significantly affected by bystander immune responses. Together, these data better define the immunological properties of VM cells and how they factor into the CD8+ T cell memory pool.

TLR signaling is carefully modulated to permit rapid innate immune responses to pathogens while avoiding excessive inflammation. Zhao et al. (p. 2567) have characterized the E3 ubiquitin ligase tripartite motif (TRIM) 38 as a negative regulator of TLR signaling in macrophages. TRIM38 expression increased significantly in macrophages upon in vitro stimulation with ligands of TLR2, TLR3, TLR4, and TLR7. Macrophages with small interfering RNA-mediated knockdown of TRIM38 had greater proinflammatory cytokine production relative to macrophages with normal TRIM38 expression, suggesting that TRIM38 is a negative regulator of TLR signaling. Additional data indicated that TRIM38 negatively regulates TLR-driven NF-κB and MAPK signaling. TRIM38 was able to bind to TNFR-associated factor 6 (TRAF6), an intracellular molecule involved in TLR signaling. The E3 ubiquitin ligase activity of TRIM38 promoted ubiquitination and proteasomal degradation of TRAF6 and a concomitant reduction in TLR-driven proinflammatory cytokine production via NF-κB and MAPK activation. These results support a unique role for TRIM38 as a modulator of TLR signaling in macrophages.

Osteoclast (OC) function is modulated in part through signaling via the cell surface receptor TREM2 and the signaling adapter DAP12. Humans with mutations that inactivate TREM2 or DAP12 may suffer from Nasu-Hakola disease (NHD), which can lead to osteoporosis. Otero et al. (p. 2612) studied TREM2−/− mice to better understand the immunological mechanisms that contribute to NHD-related osteoporosis. Despite similar levels of osteoblast function and bone formation in TREM2−/−and wild-type (WT) mice, TREM2−/− mice were significantly more osteopenic, which suggested abnormal OC function. In vitro, TREM2 deficiency correlated with reduced β-catenin expression and impaired osteoclast precursor (OcP) proliferation in response to M-CSF, which was associated with accelerated osteoclastogenesis. Conditional deletion of β-catenin in OcP caused reduced proliferation and greater osteoclastogenesis in vitro and increased osteopenia in vivo relative to WT controls. Strikingly, mice that were doubly heterozygous for TREM2 and β-catenin (βcatfl/+TREM2−/+) had significant osteoporosis compared with WT control mice. These results indicate that TREM2 and β-catenin are critical factors that regulate osteoclastogenesis and bone homeostasis, thus providing novel mechanistic insight into the cause of osteoporosis in NHD.

Tumor growth is typically accompanied by formation of leaky blood vessels lined with abnormal endothelial cells, which impairs the recruitment of antitumor T cells. TNF-α is a proinflammatory cytokine with potent antitumor activity but cannot be administered systemically due to its toxicity. Recent studies have shown that fusion of TNF-α to a tumor homing peptide (NGR-TNF) significantly reduces its systemic toxicity while enhancing its antitumor activity. Calcinotto et al. (p. 2687) show that NGR-TNF exerts multiple effects on endothelial cells lining tumor vessels, promoting the recruitment of antitumor T cells. Using a mouse model of melanoma, they observed that infusion of NGR-TNF promoted a rapid increase in adhesion molecule expression and release of cytokines and chemokines from endothelial cells lining tumor blood vessels. A significant number of tumor-infiltrating lymphocytes, especially T cells, were also detected in tumors of mice infused with NGR-TNF and were more abundant than in tumors from mice infused with control PBS or TNF. NGR-TNF treatment enhanced the effectiveness of adoptively transferred antitumor T cells with respect to shrinking tumors and prolonging survival. Vaccination with antitumor peptide-pulsed dendritic cells alone or in combination with chemotherapy was also enhanced by NGR-TNF infusion. Taken together, these results indicate that targeting TNF to the vascular endothelium of tumor cells can enhance various antitumor therapies.

IL-15 has been shown to influence the development and homeostasis of memory CD8+ T cells, but its effect on responses restricted by nonconventional MHC class Ib (MHC 1b) molecules is not well understood. Müller et al. (p. 2575) show that CD8+ T cell responses to Listeria monocytogenes restricted by Ags presented on the MHC 1b molecule H2-M3 were selectively dependent on IL-15. The development and cytolytic activity of H2-M3–restricted L. monocytogenes-specific CD8+ T cell responses were significantly reduced in IL-15−/− or IL-15Rα−/− mice relative to wild-type controls in response to infection. IL-15 was needed for survival of naive H2-M3–restricted CD8+ T cells as well as the majority of naive CD8+ T cells restricted by different MHC 1b molecules, but was not required for proliferation or survival of activated H2-M3–restricted CD8+ T cells. H2-M3–restricted, but not all MHC 1b-restricted, CD8+ T cell responses were diminished during L. monocytogenes infection in MHC 1a-deficient IL-15−/− mice. Additional data showed that both IL-15–independent and IL-15–dependent MHC 1b-restricted CD8+ T cells emerge during thymic selection, thus providing evidence of the origin of IL-15–independent MHC 1b-restricted CD8+ T cells that respond to L. monocytogenes. These data suggest that the nature of the MHC 1b molecule impacts the IL-15 dependency of CD8+ T cells during thymic selection.

Summaries written by Christiana N. Fogg, Ph.D.