Infections induce pathogen-specific T cell differentiation into diverse effectors (Teff) that give rise to memory (Tmem) subsets. The cell-fate decisions and lineage relationships that underlie these transitions are poorly understood. Here, we found that the chemokine receptor CX3CR1 identifies three distinct CD8+ Teff and Tmem subsets. Classical central (Tcm) and effector memory (Tem) cells and their corresponding Teff precursors were CX3CR1- and CX3CR1high, respectively. Viral infection also induced a numerically stable CX3CR1int subset that represented ∼15% of blood-borne Tmem cells. CX3CR1int Tmem cells underwent more frequent homeostatic divisions than other Tmem subsets and not only self-renewed, but also contributed to the expanding CX3CR1- Tcm pool. Both Tcm and CX3CR1int cells homed to lymph nodes, but CX3CR1int cells, and not Tem cells, predominantly surveyed peripheral tissues. As CX3CR1int Tmem cells present unique phenotypic, homeostatic, and migratory properties, we designate this subset peripheral memory (tpm) cells and propose that tpm cells are chiefly responsible for the global surveillance of non-lymphoid tissues.

Cover by Carmen Gerlach

Neutrophils are constantly generated from hematopoietic stem and progenitor cells in the bone marrow to maintain high numbers in circulation. A considerable number of neutrophils and their progenitors have been shown to be present in the spleen too; however, their exact role in this organ remains unclear. Herein, we sought to study the function of splenic neutrophils and their progenitors using a mouse model for sterile, peritoneal inflammation. In this microcapsule device implantation model, we show chronic neutrophil presence at implant sites, with recruitment from circulation as the primary mechanism for their prevalence in the peritoneal exudate. Furthermore, we demonstrate that progenitor populations in the spleen play a key role in maintaining elevated neutrophil numbers. Our results provide new insight into the role for splenic neutrophils and their progenitors and establish a model to study neutrophil function during sterile inflammation.

Systemic sclerosis (SSc) is a potentially fatal autoimmune disorder with limited therapeutic options. Sclerodermatous graft versus host disease (sclGvHD), induced by transfer of B10.D2 splenocytes into BALB/c Rag2-/- mice, models an inflammatory subset of SSc characterized by a prominent IL13-induced gene expression signature in the skin. Host mice deficient in IL4RA, a subunit of the type II IL4/IL13 receptor, are protected from sclGvHD. While IL4RA has a well-established role in Th2 differentiation and alternative macrophage activation, we report here a previously unappreciated function for IL4RA in lymphatic endothelial cells (LECs): regulation of activated T cell egress. Seven days after splenocyte transfer, Il4ra-/- hosts had increased numbers of activated graft CD4+ T cells in skin draining lymph nodes (dLNs) but fewer T cells in efferent lymph, blood, and skin. Sphingosine-1 phosphate (S1P), master regulator of lymphocyte egress from LNs, was lower in dLNs of Il4ra-/- hosts with a corresponding decrease of S1P kinase 1 (Sphk1) expression in LECs. Bypassing the efferent lymphatics via i.v. injection of CD4+ T cells from dLNs of Il4ra-/- sclGvHD mice restored clinical GvHD in secondary Il4ra-/- recipients. These results identify a role for IL4RA and suggest that modulation of lymphocyte egress from LNs may be effective in SSc and GvHD.

Heterogeneity and functional specialization among skin-resident macrophages are incompletely understood. In this study, we describe a novel subset of murine dermal perivascular macrophages that extend protrusions across the endothelial junctions in steady-state and capture blood-borne macromolecules. Unlike other skin-resident macrophages that are reconstituted by bone marrow-derived progenitors after a genotoxic insult, these cells are replenished by an extramedullary radio-resistant and UV-sensitive Bmi1(+) progenitor. Furthermore, they possess a distinctive anti-inflammatory transcriptional profile, which cannot be polarized under inflammatory conditions, and are involved in repair and remodeling functions for which other skin-resident macrophages appear dispensable. Based on all their properties, we define these macrophages as Skin Transendothelial Radio-resistant Anti-inflammatory Macrophages (STREAM) and postulate that their preservation is important for skin homeostasis.

Natural killer (NK) cells are critical players against tumors. The outcome of anti-tumor vaccination protocols depends on the efficiency of NK-cell activation, and efforts are constantly made to manipulate them for immunotherapeutic approaches. Thus, a better understanding of NK-cell activation dynamics is needed. NK-cell interactions with accessory cells and trafficking between secondary lymphoid organs and tumoral tissues remain poorly characterized. Here, we show that upon triggering innate immunity with lipopolysaccharide (LPS), NK cells are transiently activated, leave the lymph node, and infiltrate the tumor, delaying its growth. Interestingly, NK cells are not actively recruited at the draining lymph node early after LPS administration, but continue their regular homeostatic turnover. Therefore, NK cells resident in the lymph node at the time of LPS administration become activated and exert anti-tumor functions. NK-cell activation correlates with the establishment of prolonged interactions with dendritic cells (DCs) in lymph nodes, as observed by two-photon microscopy. Close DC and NK-cell contacts are essential for the localized delivery of DC-derived IL-18 to NK cells, a strict requirement in NK-cell activation.

Tumor-derived extracellular vesicles (tEVs) are important signals in tumor-host cell communication, yet it remains unclear how endogenously produced tEVs affect the host in different areas of the body. We combined imaging and genetic analysis to track melanoma-derived vesicles at organismal, cellular, and molecular scales to show that endogenous tEVs efficiently disseminate via lymphatics and preferentially bind subcapsular sinus (SCS) CD169(+) macrophages in tumor-draining lymph nodes (tdLNs) in mice and humans. The CD169(+) macrophage layer physically blocks tEV dissemination but is undermined during tumor progression and by therapeutic agents. A disrupted SCS macrophage barrier enables tEVs to enter the lymph node cortex, interact with B cells, and foster tumor-promoting humoral immunity. Thus, CD169(+) macrophages may act as tumor suppressors by containing tEV spread and ensuing cancer-enhancing immunity.

Active-targeted delivery to lymph nodes represents a major advance toward more effective treatment of immune-mediated disease. The MECA79 antibody recognizes peripheral node addressin molecules expressed by high endothelial venules of lymph nodes. By mimicking lymphocyte trafficking to the lymph nodes, we have engineered MECA79-coated microparticles containing an immunosuppressive medication, tacrolimus. Following intravenous administration, MECA79-bearing particles showed marked accumulation in the draining lymph nodes of transplanted animals. Using an allograft heart transplant model, we show that targeted lymph node delivery of microparticles containing tacrolimus can prolong heart allograft survival with negligible changes in tacrolimus serum level. Using MECA79 conjugation, we have demonstrated targeted delivery of tacrolimus to the lymph nodes following systemic administration, with the capacity for immune modulation in vivo.