Mast cells (MCs) are strategically distributed at barrier sites and prestore various immunocyte-recruiting cytokines, making them ideal targets for selective activation to treat peripheral infections. Here, we report that topical treatment with mastoparan, a peptide MC activator (MCA), enhances clearance of Staphylococcus aureus from infected mouse skins and accelerates healing of dermonecrotic lesions. Mastoparan functions by activating connective tissue MCs (CTMCs) via the MRGPRX2 (Mas-related G protein-coupled receptor member X2) receptor. Peripheral CTMC activation, in turn, enhances recruitment of bacteria-clearing neutrophils and wound-healing CD301b⁺ dendritic cells. Consistent with MCs playing a master coordinating role, MC activation also augmented migration of various antigen-presenting dendritic cells to draining lymph nodes, leading to stronger protection against a second infection challenge. MCAs therefore orchestrate both the innate and adaptive immune arms, which could potentially be applied to combat peripheral infections by a broad range of pathogens.

Dendritic cells give mast cells a nudge: Anaphylaxis is a life-threatening allergic reaction triggered after antigen-specific immunoglobulin E (IgE) antibodies bind to target allergens. These antibodies then cross-link IgE-specific Fc receptors on the surface of mast cells. The mast cells rapidly release inflammatory mediators, including histamine, resulting in smooth muscle contraction, vasodilation, and blood vessel leakage. Because mast cells are usually found in the perivascular abluminal surface of blood vessels, it has been unclear how blood-borne allergens can interact with them. Choi et al. used live intravital imaging of the mouse vasculature to show that a specialized subset of dendritic cells sample blood-borne antigens and relay them to mast cells on the surface of microvesicles (see the Perspective by Levi-Schaffer and Scheffel). IgE-bound mast cells then vigorously degranulate after contact with these microvesicles. Science , this issue p. eaao0666 ; see also p. 640

Activated platelets cause microvascular inflammation via direct activation of perivascular mast cells.

Many urinary tract infections (UTIs) are recurrent because uropathogens persist within the bladder epithelial cells (BECs) for extended periods between bouts of infection. Because persistent uropathogens are intracellular, they are often refractive to antibiotic treatment. The recent discovery of endogenous Lactobacillus spp. in the bladders of healthy humans raised the question of whether these endogenous bacteria directly or indirectly impact intracellular bacterial burden in the bladder. Here, we report that in contrast to healthy women, female patients experiencing recurrent UTIs have a bladder population of Lactobacilli that is markedly reduced. Exposing infected human BECs to L. crispatus in vitro markedly reduced the intracellular uropathogenic Escherichia coli (UPEC) load. The adherence of Lactobacilli to BECs was found to result in increased type I interferon (IFN) production, which in turn enhanced the expression of cathepsin D within lysosomes harboring UPECs. This lysosomal cathepsin D–mediated UPEC killing was diminished in germ-free mice and type I IFN receptor–deficient mice. Secreted metabolites of L. crispatus seemed to be responsible for the increased expression of type I IFN in human BECs. Intravesicular administration of Lactobacilli into UPEC-infected murine bladders markedly reduced their intracellular bacterial load suggesting that components of the endogenous microflora can have therapeutic effects against UTIs.

There is a growing consensus that a significant proportion of recurrent urinary tract infections are linked to the persistence of uropathogens within the urinary tract and their re-emergence upon the conclusion of antibiotic treatment. Studies in mice and human have revealed that uropathogenic Escherichia coli (UPEC) can persist in bladder epithelial cells (BECs) even after the apparent resolution of the infection. Here, we found that, following the entry of UPEC into RAB27b⁺ fusiform vesicles in BECs, some bacteria escaped into the cytoplasmic compartment via a mechanism involving hemolysin A (HlyA). However, these UPEC were immediately recaptured within LC3A/B⁺ autophagosomes that matured into LAMP1⁺ autolysosomes. Thereafter, HlyA⁺ UPEC-containing lysosomes failed to acidify, which is an essential step for bacterial elimination. This lack of acidification was related to the inability of bacteria-harboring compartments to recruit V-ATPase proton pumps, which was attributed to the defragmentation of cytosolic microtubules by HlyA. The persistence of UPEC within LAMP1⁺ compartments in BECs appears to be directly linked to HlyA. Thus, through intravesicular instillation of microtubule stabilizer, this host defense response can be co-opted to reduce intracellular bacterial burden following UTIs in the bladder potentially preventing recurrence.