Isotype-Specific Transcriptomes of Antibody-Secreting Cells
Whereas it is well documented that Ab isotypes have different functions, it is not known if the isotypes are produced by distinct Ab-secreting cells (ASC). As ASCs downregulate their surface BCR upon differentiation, correlating BCR characteristics to other molecular characteristics has proven difficult. To determine if transcriptional signatures are shared among ASC of distinct BCR isotypes, Price et al. (p. 2121) developed a fixation and staining protocol to identify influenza-responding ASC of distinct BCR subtypes, followed by fixation to allow RNA-sequencing of cell subsets. Although a shared ASC signature was identified, each isotype-specific population expressed distinct transcriptional programs. IgA ASC expressed high levels of genes important for homing to mucosal sites. IgM and IgG ASC expressed significantly high levels of Cxcr4, which is important for establishing long-lived ASC in the BM, whereas IgG ASC also expressed Cxcr3, which directs Ag-specific cells to the lung. Furthermore, IgA and IgM ASC expressed high levels of Tnfrsf, which encodes B cell maturation Ag, and IgG ASC expressed high levels of Ada, which encodes adenosine deaminase, an enzyme involved in purine metabolism. Finally, analysis of BCR clonotypes demonstrated that each ASC isotype contained a unique, clonally related CDR3 repertoire. Together, these data reveal unique features of ASC subsets that correlate with functional differences and may be critical in vaccine design when one isotype must be favored over another.
Do Aged Alveolar Macrophages Know New Tricks?
Elderly patients are more susceptible to pulmonary infections, including tuberculosis (TB). To better understand how aging changes innate cells of the lung, Lafuse et al. (p. 2252) studied cytokine profiles, activation marker expression, and susceptibility to TB in alveolar macrophages (AMs) of young (3 mo) and old (18 mo) mice. The authors identified two distinct populations of AMs present in both cohorts, a large CD11c+ CD11b− population and a smaller CD11c+ CD11b+ population. Interestingly, the percentage of CD11c+ CD11b+ AMs in the aged mice was 4-fold higher when compared with the young mice. CD11c+ CD11b+ AMs were determined to be monocyte derived, based on their expression of Ly6C, CX3CR1, and CD115. Independent of age, increased expression of CD206, CD86, and CD36 on CD11c+ CD11b+ AMs suggested that they are in a more activated state than the CD11c+ CD11b− AMs. Additionally, CD11c+ CD11b+ AMs expressed higher levels of inflammatory cytokines IL-1β, IL-6, and cyclooxygenase 2 and the chemokine CCL2, whereas CD11c+ CD11b− AMs expressed higher levels of the immune-regulatory cytokines IFN-β and IL-10. Additionally, CD11c+ CD11b− AMs in aged mice infected with Mycobacterium tuberculosis were more permissive for bacterial growth and survival. Together, these data highlight changes associated with age in innate lung cells and the need for more studies to better understand how these changes can impact chronic inflammation or immunity.
Suppressors of Osteoclastogenesis
Recent studies have suggested that forkhead box class O 3 (Foxo3) plays a role in osteoclastogenesis and rheumatoid arthritis (RA). In this issue, Xu et al. (p. 2141) investigated the biological function of the FOXO locus during osteoclastogenesis. Mice deficient in Foxo3 displayed enhanced receptor activator for NF-κB ligand (RANKL)–induced osteoclastogenesis, indicating that Foxo3 acts as a negative regulator of osteoclast differentiation. Deletion of Foxo3 in myeloid lineage osteoclast precursors revealed an N-terminal truncated Foxo3 protein that is an ortholog of the human FOXO3 isoform2. Expression of Foxo3 isoform2 in bone marrow–derived macrophages or RAW264.7 cells suppressed osteoclast differentiation induced by RANKL. Furthermore, mice expressing Foxo3 isoform2 displayed an osteopetrotic phenotype, further supporting a role for Foxo3 isoform2 as an inhibitor of bone regeneration. Mechanistically, Foxo3 isoform2 was found to enhance the expression of type-I IFN response genes to RANKL stimulation, thereby inhibiting osteoclastogenesis via IFN-β–mediated feedback inhibition. Therefore, this study identifies the biological function of Foxo3 isoform2 as a suppressor of osteoclast differentiation, which may be of particular interest for the treatment of RA or osteoporosis.
IL-10 Balance in Sepsis
Interleukin-10 has long been considered an anti-inflammatory cytokine. However, new studies have demonstrated that IL-10 may also have proinflammatory properties. In this issue, Mazer et al. (p. 2088) investigate the role of IL-10 in the delicate cytokine balance of sepsis. Septic patients had significantly higher serum levels of IL-10 than critically ill nonseptic (CINS) patients. Ex vivo inhibition of IL-10 in PMBCs from septic patients increased both T cell expression of IFN-γ and monocyte expression of TNF-α. Conversely, ex vivo treatment of PBMCs with human rIL-10 (rhIL-10) increased IFN-γ in only the septic patients, whereas TNF-α was decreased in all patients. Additionally, overnight stimulation of PMBCs with rhIL-10 significantly decreased HLA-DR expression in all patients. These results were confirmed in a murine model of sepsis in which IL-10 treatment also increased IFN-γ production. These data provide further evidence that IL-10 may act in an immunostimulatory capacity and that its effects may depend on cellular environment, as well as disease state.
Effector CD8 T Cells with Innate-like Properties
Chronic infections, such as untreated HIV-1, can induce the expansion of late-stage differentiated effector CD8 T cells that may persist long after viral escape from TCR recognition. In this issue, Naluyima et al. (p. 2210) sought to determine if these CD8 T cell populations can perform unconventional innate-like antiviral effector functions. Chronic, untreated HIV-1 infection was associated with elevated numbers of terminal CD8 effector cells expressing FcγRIIIA (CD16). These cells had a distinct transcriptional profile characterized by high levels of IKZF2 and low expression of IL-7R, which translated into an NKp80+ IL-7Rα− surface phenotype with high expression of the Helios transcription factor. Functionally, FcγRIIIA+ CD8 T cells were highly active in a manner that positively correlated with plasma levels of soluble mediators of antiviral immunity and inflammation, such as IP-10, TNF, IL-6, and TNFRII. These cells also mediated Ab-dependent cellular cytotoxicity (ADCC) activity at levels similar to NK cells on a per cell basis. Importantly, the frequency of FcγRIIIA+ CD8 T cells persisted as patients initiated suppressive antiretroviral therapy, although their activation levels declined. Therefore, these data indicate that terminally differentiated effector CD8 T cells acquire enhanced innate-like properties during chronic viral infection, characterized by virus-specific ADCC, which may be targeted to treat HIV infection.
Tolerance for Self-Reactive B Cells
B cells expressing the AM14 BCR derived from an autoimmune MRL/lpr mouse only proliferate in response to IgG2a immune complexes (ICs) that incorporate DNA, RNA, or nucleic acid binding proteins, which serve as autoantigens. Therefore, these cells are a useful tool for modeling the importance of BCR/TLR coengagement in the activation of autoreactive B cells. To better define receptor-specific signaling cascades in this model, Nündel et al. (p. 2055) backcrossed AM14 and Vκ8 transgenes to C57BL/6J (B6) mice to use tissue-specific deletion strains only available on a B6 background. The authors demonstrated that the AM14 Vκ8 BCR recognized a B6-derived cross-reactive Ag with sufficient avidity to induce a TLR-independent proliferative response of BALB/c AM14 Vκ8 B cells both in vitro and in vivo. Detection of this cross-reactive Ag by B6 AM14 Vκ8 B cells promoted an anergic phenotype, as characterized by the absence of mature B cells in the bone marrow, and suboptimal responses to BCR crosslinking by IgM. This B6 Ag also impacted B cell development as evidenced by an expanded marginal zone B cell compartment, and extensive receptor editing in B6 but not BALB/C AM14 Vκ8 mice. Interestingly, anergic B6 AM14 Vκ8 B cells were able to fully activate in response to autoantigen/autoadjuvant ICs. These data, therefore, provide additional insight about tolerance for self-reactive B cells in situations when the self-reactive Ag is expressed on peripheral but not bone marrow cells.