Inflammation stimulates lymphatic endothelial cells to release exosomes, which accumulate in the perivascular stroma. Brown et al. show that these exosomes promote the directional migration of dendritic cells along guidance cues in complex environments by enhancing dynamic cellular protrusions in a CX3CL1-dependent manner.

TYK2 is a member of the JAK family of tyrosine kinases that is involved in chromosomal translocation-induced fusion proteins found in anaplastic large cell lymphomas (ALCL) that lack rearrangements activating the anaplastic lymphoma kinase (ALK). Here we demonstrate that TYK2 is highly expressed in all cases of human ALCL, and that in a mouse model of NPM-ALK-induced lymphoma, genetic disruption of Tyk2 delays the onset of tumors and prolongs survival of the mice. Lymphomas in this model lacking Tyk2 have reduced STAT1 and STAT3 phosphorylation and reduced expression of Mcl1, a pro-survival member of the BCL2 family. These findings in mice are mirrored in human ALCL cell lines, in which TYK2 is activated by autocrine production of IL-10 and IL-22 and by interaction with specific receptors expressed by the cells. Activated TYK2 leads to STAT1 and STAT3 phosphorylation, activated expression of MCL1 and aberrant ALCL cell survival. Moreover, TYK2 inhibitors are able to induce apoptosis in ALCL cells, regardless of the presence or absence of an ALK-fusion. Thus, TYK2 is a dependency that is required for ALCL cell survival through activation of MCL1 expression. TYK2 represents an attractive drug target due to its essential enzymatic domain, and TYK2-specific inhibitors show promise as novel targeted inhibitors for ALCL.

Extracellular vesicles, released from cells, are important for intercellular communication. They are heterogeneous but fall into two broad categories based on origin and function: microvesicles formed by outward budding from the plasma membrane; and exosomes that originate as intraluminal vesicles in multivesicular endosomes that fuse with the plasma membrane to release them. Extracellular vesicles generally and exosomes in particular have powerful effects on specific immune responses, and recent advances highlight their potential therapeutic uses. Dendritic cells (DC) that have internalized antigen release exosomes that express MHC class II molecules loaded with antigenic peptides, co-stimulatory molecules and intact antigen. Depending on the setting, these stimulate CD4 T-cell proliferation either directly or only in the context of accessory antigen naïve DC. Here, we discuss the reasons for this; and review current knowledge about the loading of antigen, class II and other cargo into exosomes released by DC and other professional antigen-presenting cells in the context of advances in exosome biology more generally.

The lysosome-associated membrane protein (LAMP) family includes the dendritic cell endocytic receptors DC-LAMP and CD68, as well as LAMP-1 and LAMP-2. In this study we identify LAMP-1 (CD107a) and LAMP-2 (CD107b) on the surface of human monocyte-derived dendritic cells (MoDC) and show only LAMP-2 is internalized after ligation by specific Abs, including H4B4, and traffics rapidly but transiently to the MHC class II loading compartment, as does Ag conjugated to H4B4. However, pulsing MoDC with conjugates of primary (keyhole limpet hemocyanin; KLH) and recall (Bet v 1) Ags (H4B4*KLH and H4B4*Bet v 1) induced significantly less CD4 cell proliferation than pulsing with native Ag or Ag conjugated to control mAb (ISO*KLH and ISO*Bet v 1). In H4B4*KLH-pulsed MoDC, the duration of KLH residence in MHC class II loading compartments was significantly reduced, as were surface HLA-DR and DR-bound KLH-derived peptides. Paradoxically, MoDC pulsed with H4B4*KLH, but not the other KLH preparations, induced robust proliferation of CD4 cells separated from them by a transwell membrane, indicating factors in the supernatant were responsible. Furthermore, extracellular vesicles from supernatants of H4B4*KLH-pulsed MoDC contained significantly more HLA-DR and KLH than those purified from control MoDC, and KLH was concentrated specifically in exosomes that were a uniquely effective source of Ag in standard T cell proliferation assays. In summary, we identify LAMP-2 as an endocytic receptor on human MoDC that routes cargo into unusual Ag processing pathways, which reduces surface expression of Ag-derived peptides while selectively enriching Ag within immunogenic exosomes. This novel pathway has implications for the initiation of immune responses both locally and at distant sites.

Although great advances have been made in understanding the pathobiology of MLL-rearranged (MLL-r) leukemias, therapies for this leukemia have remained limited, and clinical outcomes remain bleak. To identify novel targets for immunotherapy treatments, we compiled a lineage-independent MLL-r leukemia gene signature using publicly available data sets. Data from large leukemia repositories were filtered through the In-silico Human Surfaceome, providing a list of highly predicted cell surface proteins overexpressed in MLL-r leukemias. LAMP5, a lysosomal associated membrane protein, is expressed highly and specifically in MLL-r leukemia. We found that LAMP5 is a direct target of the oncogenic MLL-fusion protein. LAMP5 depletion significantly inhibited leukemia cell growth in vitro and in vivo. Functional studies showed that LAMP-5 is a novel modulator of innateimmune pathways in MLL-r leukemias. Downregulation of LAMP5 led to inhibition of NF-κB signaling and increased activation of type-1 interferon signaling downstream of Toll-like Receptor/Interleukin 1 Receptor activation. These effects were attributable to the critical role of LAMP-5 in transferring the signal flux from Interferon Signaling Endosomes to Pro-Inflammatory Signaling Endosome. Depletion of IRF7 was able to partially rescue the cell growth inhibition upon LAMP5 downregulation. Lastly, LAMP-5 was readily detected on the surface of MLL-r leukemia cells. Targeting surface LAMP-5 using an antibody-drug conjugate leads to significant cell viability decrease specifically on MLL-r leukemias. Overall, based on the limited expression throughout human tissues, we postulate that LAMP-5 could potentially serve as an immunotherapeutic target with a wide therapeutic window to treat MLL-r leukemias.