Aging T Cells Can’t Chew the Fat See article p. 562

Proinflammatory Effects of Inactive IDO2 See article p. 571

LAG-3 and Autoimmunity See article p. 594

BCL-2 Promotes Survival of Intraepithelial Lymphocyte Precursors See article p. 651

Rhesus Macaque Ag Receptor Repertoires See article p. 762

Engineered IgE for Clinical Assay Standardization See article p. 772

As people age, their CD8+ T cells become less efficient at responding to novel Ags, leaving the elderly more susceptible to novel pathogens and less responsive to vaccines. In this Top Read, Nicoli et al. (p. 562) investigated the role of lipid metabolism in aged CD8+ T cell responses. The authors compared mitochondrial health, glucose and lipid uptake, susceptibility to apoptotic death, and TCR activation potential of CD8+ T cells isolated from the circulation of healthy, middle-aged (19–55 y old) and elderly (65–95 y old) adults. Both middle-aged and elderly CD8+ T cells responded to TCR activation, although the elderly T cells showed lower proliferation rates and higher incidence of apoptosis after activation. Relative to unstimulated naive, middle-aged cells, the elderly counterparts demonstrated higher basal expression of T-bet and CD134, both markers of activated T cells, suggesting high basal levels of activation. Indeed, these features correlated with higher mitochondrial membrane potential and increased expression of lipid and fatty acid storage genes. However, no difference in expression of glycolytic genes was detected. The authors were able to inhibit activation-induced apoptosis and restore TCR-stimulated proliferation by treating elderly CD8+ T cells with agents that promote lipid catabolism. Together, the study suggests that therapeutically reversing age-acquired lipid and fatty acid storage defects in CD8+ T cells may restore T cell functionality for the elderly population, protecting them from pathogenic assaults and enhancing their vaccine response.

Cataloguing the heterogeneous lymphocyte receptors repertoire is technically difficult but critical to understanding the adaptive immune responses to specific immune assaults. In this Top Read, Walsh et al. (p. 762) presented a method to transform a single-cell immune repertoire sequencing (scIRS) assay into a high-throughput procedure that could successfully catalog the diversity among rhesus macaque B and TCRs. Using their droplet-based single-cell technology to assay tens of thousands of rhesus lymphocytes, every known Ig isotype and TCR chain type was recovered. In addition, this method provided a workflow for sequencing and clustering thousands of individual lymphocytes and overlaying their gene expression profiles with V(D)J repertoires to detect clonally expanded lineages. Interestingly, one type of rare, hybrid lymphocyte was detected, wherein both Ig and TCR genes are expressed. Altogether, the data and analysis validate the use of scIRS to thoroughly catalog immune repertoires, and thus improves our understanding of the development of adaptive immune responses in rhesus macaques.

In this Top Read, Merlo et al. (p. 571) showed that the enzymatic activity of tryptophan catabolizing enzyme, IDO2, is not required for its proinflammatory effects in autoimmune arthritis. The authors created two lines of knock-in mice (R235W and Y346X) with common point mutations that result in loss of IDO2 enzymatic activity. Protein levels in the R235W mice were similar to wild-type (WT) levels, whereas Y346X mice showed less IDO2 protein, likely due to the instability of the point mutant. In a preclinical model, R235W mice developed robust arthritis, whereas Y346X mice had attenuated joint inflammation. Adoptive transfer experiments confirmed that the nonenzymatic IDO2 from B cells mediated disease progression. Indeed, R235W mice had similar levels of autoreactive B cells as WT mice, whereas autoreactive B cell numbers in Y346X mice were similar to total IDO2 knockout mice. The authors also showed that GAPDH, Runx1, RANbp10, and Mgea15 bind IDO2, as well as the two nonenzymatic mutants, with varying affinities. Together, these data show that IDO2 contributes to the proinflammatory pathology of autoimmune arthritis independent of its enzymatic activity.

Lymphocyte activation gene 3 (LAG-3) is a coinhibitory receptor on T lymphocytes that negatively regulates their activation and proliferation. In this Top Read, Jones et al. (p. 594) showed that patients with autoimmune diseases, such as relapsing-remitting multiple sclerosis (RRMS) or type 1 diabetes, have fewer LAG-3+ T cells compared with healthy control subjects. The low levels of functional LAG-3 protein on T cells from RRMS patients may be a result of altered mRNA expression. The authors found that LAG-3 expression is associated with cell death markers Fas and cleaved caspase 3. Therefore, the low expression of LAG-3 on T cells in RRMS patients may indicate that these T cells are more resistant to cell death in vivo than LAG-3+ T cells. Such profiles may explain how autoreactive T cells escape negative selection and avoid apoptosis in autoimmunity patients. Collectively, the authors suggest that a LAG-3 agonist may be effective in treating autoimmune conditions driven by autoreactive T cells, including RRMS and type 1 diabetes.

Immunoglobulin E is a necessary standard for many clinical diagnostic tests. However, current methods of producing IgE may lead to cross-reactivity from variable domains in the H and L chains. In this Top Read, Kim et al. (p. 772) demonstrated that their engineered, recombinant Ig fragment (IgCw-γεκ), comprised of two hybrid constant H chain regions (Cγ1-Cε2–4) and two constant κ L chains (Cκ), is a biologically active alternative to IgE. Transfection of human embryonic kidney cells with either IgE or IgCw-γεκ resulted in similar protein yields. IgCw-γεκ bound human FcεRIα with an affinity similar to native IgE, and inhibited in vitro sensitization of basophils by IgE, suggesting structural and functional similarities are retained in the engineered version. Finally, the authors showed that IgCw-γεκ can be used in ELISA as an accurate reference standard. Together, these data provide support for the use of IgCw-γεκ as an alternative for IgE in laboratory and diagnostic assays.

Intraepithelial lymphocytes (IEL) are found in mucosal tissues such as the gastrointestinal tract. In this Top Read, Shanmuganad et al. (p. 651) distinguished the roles of the proapoptotic Bim and the antiapoptotic BCL-2 proteins during IEL precursor (IELp) survival. Using conditional knockout mice, they showed that BCL-2 promotes the survival of thymic IELps by antagonizing Bim. Survival of α4β7+ and Runx3+ IELps was most dependent on BCL-2 compared with other IELp populations. Loss of thymic BCL-2 eliminated most gut IELs, but this phenotype could be reversed upon concurrent deletion of thymic Bim. BCL-2 is required for thymic IELp seeding of the gut and for peripheral IEL survival. These data demonstrate that the survival of IELps and peripheral IELs depends on differential temporal and spatial expression for BCL-2 and Bim.