In This Issue

Effector and memory CD8⁺ T cells can be subdivided into multiple subsets with distinct functional characteristics. Jung et al. (p. 5315) analyzed the localization of effector and memory T cell subsets in the spleen during acute lymphocytic choriomeningitis virus infection to gain a better understanding of T cell subset homeostasis and function. Most of the long-lived KLRG1^loIL-7R^hi memory precursor effector CD8⁺ T cells were found in the splenic T cell area, whereas the shorter-lived KLRG1^hiIL-7R^lo effector CD8⁺ T cells were located in the red pulp. Migration of Ag-specific KLRG1^loIL-7R^hi memory precursor cells into the T cell zone appeared to be specifically directed by CCR7, whereas both KLRG1^loIL-7R^hi and KLRG1^hiIL-7R^lo cells responded to the CXCR4 ligand CXCL12. Expression of Ccr7 and consequent T cell localization was modulated by the transcription factors T-bet and Blimp-1. In the T cell zone, the memory precursor effector cells were found in close association with gp38⁺ stromal cells, suggesting the delivery of survival signals necessary for memory development. Analysis of memory T cell subsets demonstrated that splenic central memory T cells responded to CCR7 ligands and homed to the T cell zone, whereas effector memory T cells responded to CXCR4 ligands and homed to the red pulp. These data demonstrate differentiation state-dependent T cell migration within lymphoid organs and provide insight into the establishment of long-lived memory CD8⁺ T cell responses to viral infection.

Myeloid-derived suppressor cells (MDSCs) produce arginase I, leading to L-arginine (L-Arg) depletion. In T cells, this depletion causes an arrest of the cell cycle at the G₀-G₁ phase that is associated with reductions in cyclin D3 mRNA stability and translation. To better understand the mode of action of MDSCs, Rodriguez et al. (p. 5198) sought to identify the mechanisms involved in the inhibition of cyclin D3 mRNA stability. Elements in the 3′-untranslated region (UTR) of the cyclin D3 mRNA were found to control decreased mRNA stability in the absence of L-Arg. The stability-promoting RNA-binding protein HuR bound to the cyclin D3 3′-UTR in activated T cells only in the presence of L-Arg, and small interfering RNA-mediated knockdown of HuR decreased cyclin D3 mRNA stability and protein expression. L-Arg depletion led to decreased HuR translation that occurred due to a global inhibition of de novo protein synthesis requiring general control nonderepressible 2 (GCN2) kinase. These results provide a mechanism by which MDSC-mediated L-Arg deprivation suppresses T cell proliferation and propose that GCN2 may be a central mediator of T cell cycle inhibition.

The complement and coagulation systems are complex inflammatory systems involving proteolytic cascades of serine proteases that share many similar features. Interactions between the cascades have been reported, but the mechanisms underlying these connections are not yet clear. Amara et al. (p. 5628) analyzed molecular interactions between several components of these pathways and found that the two networks may be more closely intertwined than previously anticipated. Thrombin, coagulation factor Xa, and plasmin were all found to cleave complement components C3 and C5 in vitro to generate the anaphylotoxins C3a and C5a. Factor Xa also activated the complement system in human serum ex vivo, as demonstrated by the generation of C3a, C5a, and the terminal complement complex. Clinical relevance of these data was suggested by the demonstration of a correlation between coagulation and C5a production in the plasma of polytrauma patients. The authors propose that the coagulation/fibrinolysis cascades and the complement system are intimately linked such that it is appropriate to consider them as a single “serine protease network” rather than as separate cascades.

A rapid and severe depletion of intestinal CD4⁺ T cells following HIV infection leads to mucosal immune dysfunction and microbial access to the circulation. The most extensive studies of this phenomenon have been carried out in the experimental SIV model, leaving many questions regarding HIV-induced depletion unanswered. To address some of these questions, Gordon et al. (p. 5169) carried out a systematic analysis of CD4⁺ T cells in peripheral blood and four gastrointestinal (GI) tract sites in HIV-infected individuals with active viral replication, HIV-infected individuals undergoing antiretroviral therapy, and healthy control volunteers. In the blood and all four GI sites, CD4⁺ T cells were significantly depleted by active HIV infection. The level of cell depletion in the gut directly correlated with that in the blood and was partially reversed by antiretroviral therapy. HIV infection also led to memory CD4⁺ T cell proliferation in the GI tract that significantly correlated with the level of memory CD4⁺ T cell proliferation in the blood, suggesting that T cell homeostasis was similarly affected in both environments. Finally, the percentages of Th17 cells and total CD4⁺ T cells in the gut were inversely correlated with levels of CD4⁺ T cell proliferation in the periphery. This analysis of multiple gut sites from a relatively large number of individuals therefore provides data that link HIV-induced damage to the GI tract with peripheral T cell activation and will be important for future approaches to vaccine and therapy development.

Much is known about host genes involved in HIV-1 replication in vitro, but much less is understood about the factors involved in viral replication in lymphoid tissue in vivo. To address this gap in our knowledge, Smith et al. (p. 5417) examined gene expression associated with HIV-1 replication in the inguinal lymph nodes of HIV-1–infected individuals through microarray analysis. Approximately 95% of the 592 genes whose expression was significantly associated with viral load were downregulated with increasing HIV-1 replication. These genes encoded proteins involved in collagen deposition, inhibition of transcription and translation, chromatin modification, and inhibition of cell activation and proliferation, as well as several potential anti–HIV-1 host restriction factors. Interestingly, the ∼5% of genes whose expression was positively associated with viral load generally coded for proteins involved in innate and adaptive immunity, including many involved in the IFN response, which would be expected to inhibit viral replication. However, chronic immune activation was significantly associated with increasing viral load and may instead have caused enhanced viral replication. This profile of the host response to HIV-1 in lymphoid tissue provides insights into HIV-1 replication that will act as useful starting points for future research.

B cell differentiation involves transient DNA breakage during V(D)J recombination, somatic hypermutation, and class switch recombination. Cell cycle progression is tightly regulated in DNA-damaged cells by mitotic checkpoint proteins, such as MAD2. To clarify how mitotic checkpoints are regulated in B cells, Nakaya et al. (p. 5180) examined the role of the ribonucleoprotein complex protein Pcid2 in B cell differentiation. Pcid2 mRNA was expressed at high levels in pre-B, immature B, spleen transitional (T)1 B, and follicular B cells, but not in pro-B, spleen T2 B, or marginal zone B cells. Small interfering RNA-mediated knockdown of Pcid2 led to apoptosis and polyploidy, suggesting a defect in mitosis. Supporting the involvement of Pcid2 in the mitotic spindle assembly checkpoint, Pcid2 knockdown led to a specific reduction in MAD2 mRNA in the cytoplasm but did not affect the expression of other checkpoint proteins or of the cell-cycle–associated proteins cyclin A, cyclin B1, or CDK1. Conditional knockout of Pcid2 in B cells indicated that it was required for the survival of mature splenic B cells. Taken together, these data advance our understanding of the involvement of mitotic checkpoints in the regulation of Ig gene diversification and suggest differential checkpoint regulation at different stages of B cell differentiation.

Although CD8⁺ T cells are known to be involved in controlling the progression of HIV, SIV, and simian HIV (SHIV) infections, it is not clear whether this control is dominated by cytolytic mechanisms. To address this question, Balamurali et al. (p. 5093) studied acute SHIV infection in pigtail macaques and compared the dynamics of wild-type (WT) and escape mutant (EM) viruses differing at one immunodominant epitope. The authors predicted that if CD8⁺ T cell-mediated viral control was dominated by cytolysis, WT viral decay should occur more quickly than EM viral decay, as WT-infected cells would be selectively killed by CTLs. However, there were no significant differences in WT versus EM viral decay. A mathematical modeling approach predicted that, in the case of cytolytic control of SHIV infection, maximal viral escape would occur during times of rapid viral decay, whereas noncytolytic mechanisms would drive escape during viral expansion. In agreement with a predominance of noncytolytic CD8⁺ T cell activity, maximal viral escape most often occurred during rapid viral growth and correlated with CD4⁺ T cell numbers. These data challenge the common assumption that the antiviral activity of CD8⁺ T cells is mainly cytolytic and suggest that it will be important to address the noncytolytic activities of these cells in future studies of HIV pathogenesis and in vaccine development.

Hypoxia-induced mitogenic factor (HIMF; also known as FIZZ1 and RELMα) is upregulated in inflammatory lung diseases, including pulmonary hypertension (PH), pulmonary fibrosis, and allergic lung inflammation, and has mitogenic, chemotactic, and fibrinogenic activities. Yamaji-Kegan et al. (p. 5539) investigated the role of IL-4 in HIMF expression and activity in a model of hypoxia-induced PH to clarify the mechanism of HIMF action in this disease. Surprisingly, unlike in other models of lung inflammation, HIMF expression was not dependent on IL-4 or STAT6. However, IL-4 was involved in many HIMF-mediated functions in the lung, including proliferation in pulmonary arteries, extracellular matrix deposition and collagen synthesis, macrophage infiltration, and production of vascular endothelial growth factor and chemokines. IL-4Rα blockade suppressed HIMF-induced proliferation of primary pulmonary microvascular endothelial cells, but did not inhibit their migration. Together, these data indicate cooperation between the Th2 and hypoxia pathways in hypoxia-induced PH through IL-4’s control of HIMF activity in lung inflammation and vascular remodeling.

Summaries written by Jennifer Hartt Meyers, Ph.D.