In This Issue

Ninety-nine percent of follicles (oocytes) are subject to physiological degeneration and breakdown in the process of ovarian follicle atresia. Dysregulation of this process leads to the accumulation of catabolic waste (including reactive oxygen species and lipofuscin) and cellular detritus that can adversely affect reproductive health. In this issue, Wu et al. (p. 2140) detailed the importance in ovarian atresia of IL-33, a cytokine produced in the ovaries but better known for its functions in antiviral and Th2 immunity, using an Il33-deficient (Il33^−/−) mouse strain. They found that Il33 deficiency was associated with the accumulation of catabolic waste and abnormally degraded atretic follicles in the ovaries, beginning in pubescent mice (3–5 wk of age) and increasing as mice reached their reproductive peak (15–20 wk of age). Compared with age-matched wild-type mice, sexually mature Il33^−/− mice harbored significantly fewer total developing follicles and corpora lutea, temporary endocrine structures that produce hormones. These mice also had shortened reproductive lifespans, producing an average of 3.15 ± 0.61 litters compared with 5.90 ± 0.48 litters produced by Il33^+/− littermate controls. Failed disposal of atretic follicles in Il33^−/− mice was associated with impaired migration of phagocytic CD68⁺ macrophages into follicles, an event that usually occurs following a wave of apoptosis during mid-atresia, and reduced autophagy in follicular cells. These data provide compelling evidence that IL-33 plays an integral role in regulating the physiological removal of degraded tissue in the ovaries.

The gut is one of the earliest sites of HIV replication postinfection, and gut-resident dendritic cells (DCs) provide a reservoir for HIV replication and mediate the transinfection of CD4⁺ T cells. Gut environmental factors, such as retinoic acid (RA), influence the phenotypic programming of DCs to balance immunity and tolerance; however, it is not known how these factors affect DCs in the context of HIV infection. To investigate this issue, Guerra-Pérez et al. (p. 2415) cultured monocytic DCs (moDCs) with RA (RA-DCs) to simulate the gut environment. They found that RA treatment significantly upregulated DC expression of mucosal phenotypic markers as well as HIV coreceptors. RA-DCs produced more TGFβ-1 and CCL2 than their moDC counterparts and also expressed increased levels of mucosal vascular addressin cell adhesion molecule-1 (MAdCAM-1), an adhesion molecule not commonly known to be expressed by DCs. Compared with moDC:autologous CD4⁺ T cell cocultures, RA-DCs cocultured with CD4⁺ T cells were better able to form conjugates, and T cells in these cultures developed a regulatory T cell phenotype. To investigate whether this RA-induced shift toward an anti-inflammatory phenotype could influence the ability of DCs to spread HIV infection, the authors cultured HIV-loaded RA-DCs with CD4⁺ T cells and found that HIV replication was significantly higher in RA-DC:CD4⁺ T cell mixtures compared with moDC:CD4⁺ T cell cultures. This increase in replication efficiency was partially dependent upon increased MAdCAM-1 expression, as adding blocking Abs for this molecule to RA-DC:CD4⁺ T cell cultures reduced HIV replication levels to those seen in moDC:CD4⁺ T cell cultures. Together, these data suggest that the gut microenvironment influences DCs to promote HIV replication and infection.

Signal transduction in cells is mediated by dynamic and complex networks of enzymes, including protein kinases and phosphatases. Previous studies have demonstrated the importance of protein tyrosine kinases in regulating lymphocyte trafficking; however, it is still unclear how protein tyrosine phosphatases, such as protein tyrosine phosphatase type γ (PTPRG), which is highly expressed in human monocytes, contribute to this regulation. In this issue, Mirenda et al. (p. 2168) developed novel tools to study these processes and revealed a previously unknown role for PTPRG in human monocyte integrin activation. Receptor-like protein tyrosine phosphatases are found in the cell membrane as inactive homodimers that become active upon monomerization. To disrupt homodimerization of PTPRG, a membrane-permeable “Trojan peptide” mimicking the wedge domain of PTPRG was designed (P1-WD). Treatment of human monocytes with P1-WD increased PTPRG phosphatase activity, likely through a mechanism involving monomerization of this enzyme. Monocytes treated with P1-WD exhibited reduced adhesion to fibrinogen and ICAM-1 following exposure to a chemoattractant and reduced cell arrest and increased rolling in underflow adhesion assays due to an inhibition of LFA-1 triggering. To determine the extent of PTPRG’s influence on cell signaling networks, the authors conducted phosphoproteomic analysis following P1-WD treatment and revealed 31 proteins affected by PTPRG monomerization, particularly Src family kinases. Janus kinase 2 (JAK2), which is known to be involved in activation of integrins on human T cells, was also shown to be a target of PTPRG. Monocytes treated with the PTPRG-activating Trojan peptide showed reduced JAK2 phosphorylation, suggesting that PTPRG regulates monocyte adhesion through a mechanism involving JAK2 dephosphorylation. These studies provide new tools for the investigation of phosphatase activity and reveal previously unrecognized roles for PTPRG in mediating integrin activation and influencing leukocyte migration.

Allergies stem from immune recognition of normally benign elements in the environment. Glycoproteins belonging to group 1 pollen allergens are recognized by IgE from almost all individuals with grass pollen allergies. One well-studied group 1 allergen derived from Timothy grass, Phl p 1, is highly cross-reactive with pollens from other grasses. Longitudinal analyses of allergic patients have demonstrated that Phl p 1 is recognized by IgE early on in life, with sensitization to other allergens taking longer to develop, suggesting that Phl p 1 is important in the establishment of allergies. Madritsch et al. (p. 2069) investigated the ability of IgE from an allergic patient to bind to Phl p 1. An IgE-derived single chain Ab fragment (IgE-ScFv) library was generated from IgE-specific RNA transcripts and random L chains found in PBMCs from the patient. The IgE-ScFv library was enriched for recognition of four group 1 allergens by multiple rounds of ELISA panning. Enriched IgE-ScFvs were then tested for reactivity to group 1 allergens, and three were found to react to Phl p 1. The binding sites of two of these Ab fragments were mapped to distinct peptides on the N terminus of Phl p 1. Interestingly, all previously reported IgE epitopes on Phl p 1 are found in the C terminus of this allergen. Binding affinity of the IgE-ScFvs for the allergen, as determined by surface plasmon resonance (SPR), was in the nano- to micromolar range. Both IgE-ScFvs had H chain mutations and were found to react to allergens from other grass species. Additional SPR experiments demonstrated that these binders, along with a third IgE-ScFv recognizing the C terminus, could simultaneously bind Phl p 1, suggesting that multiple Ab molecules may simultaneously engage the allergen in vivo. Furthermore, the presence of all three Ab fragments in a competitive binding assay did not prevent IgE in sera of allergic patients from binding Phl p 1. This high density of available IgE epitopes on Phl p 1 may explain why it is such a potent and highly sensitizing allergen.

CD8⁺ T cell activation and subsequent generation of memory is influenced by the balance of negative and positive signals that T cells receive during an immune response. Mammalian target of rapamycin (mTOR), a signaling molecule that regulates T cell metabolism and promotes T cell activation, and CTLA-4, a potent negative regulator of T cell responses, are two molecules in seemingly opposing regulatory pathways whose interplay and function in CD8⁺ T cell memory responses are yet to be revealed. Inhibition of CTLA-4 is known to augment primary and memory T cell responses; however, recent work surprisingly suggests that low level inhibition of mTOR may also promote rather than hinder T cell memory generation. Pedicord et al. (p. 2089) used mice challenged with OVA-expressing Listeria monocytogenes and/or tumor cells to investigate the effects of dual inhibition of the mTOR and CTLA-4 pathways on the generation of CD8⁺ T cell memory. They found that mice treated during T cell priming with both rapamycin (a partial inhibitor of mTOR) and a CTLA-4 blocking Ab exhibited increased expansion and differentiation of Ag-specific CD8⁺ T cells into memory T cell precursors and improved CD8⁺ T cell recall responses in both L. monocytogenes infection and tumorigenesis models. This combination treatment enhanced early expansion, effector cytokine production, and metabolic function of memory CD8⁺ T cells. A more complete understanding of how activating and inhibiting signals may synergize in T cells is an important step in optimizing both cancer immunotherapy and immunization against pathogens.