Close Encounters of Lymphoid Cells and Bacteria

IL-18-induced HIF-1α in ILC3s ameliorates the inflammation of C. rodentium-induced colitis

Group 3 innate lymphoid cells (ILC3s) are vital for defending tissue barriers from invading pathogens. Hypoxia influences the production of intestinal ILC3-derived cytokines by activating HIF. Yet, the mechanisms governing HIF-1α in ILC3s and other innate RORγt+ cells during in vivo infections are poorly understood. In our study, transgenic mice with specific Hif-1a gene inactivation in innate RORγt+ cells (RAG1KO HIF-1α▵Rorc) exhibit more severe colitis following Citrobacter rodentium infection, primarily due to the inability to upregulate IL-22. We find that HIF-1α▵Rorc mice have impaired IL-22 production in ILC3s, while non-ILC3 innate RORγt+ cells, also capable of producing IL-22, remain unaffected. Furthermore, we show that IL-18, induced by Toll-like receptor 2, selectively triggers IL-22 in ILC3s by transcriptionally upregulating HIF-1α, revealing an oxygen-independent regulatory pathway. Our results highlight that, during late-stage C. rodentium infection, IL-18 induction in the colon promotes IL-22 through HIF-1α in ILC3s, which is crucial for protection against this pathogen.

Hijacking of the Host's Immune Surveillance Radars by Burkholderia pseudomallei

Burkholderia pseudomallei (B. pseudomallei) causes melioidosis, a potentially fatal disease for which no licensed vaccine is available thus far. The host-pathogen interactions in B. pseudomallei infection largely remain the tip of the iceberg. The pathological manifestations are protean ranging from acute to chronic involving one or more visceral organs leading to septic shock, especially in individuals with underlying conditions similar to COVID-19. Pathogenesis is attributed to the intracellular ability of the bacterium to ‘step into’ the host cell’s cytoplasm from the endocytotic vacuole, where it appears to polymerize actin filaments to spread across cells in the closer vicinity. B. pseudomallei effectively evades the host’s surveillance armory to remain latent for prolonged duration also causing relapses despite antimicrobial therapy. Therefore, eradication of intracellular B. pseudomallei is highly dependent on robust cellular immune responses. However, it remains ambiguous why certain individuals in endemic areas experience asymptomatic seroconversion, whereas others succumb to sepsis-associated sequelae. Here, we propose key insights on how the host’s surveillance radars get commandeered by B. pseudomallei.

A polysaccharide from natural Cordyceps sinensis regulates the intestinal immunity and gut microbiota in mice with cyclophosphamide-induced intestinal injury

A polysaccharide from Cordyceps sinensis (NCSP) was reported to attenuate intestinal injury and regulate the balance of T helper (Th)1/Th2 cells in immunosuppressed mice. However, whether it influences Th17 and regulatory T (Treg) cells as well as gut ecology remains unknown. In the present study, the intestinal injury mouse model was also established by intraperitoneal injection of cyclophosphamide (Cy) for three consecutive days. NCSP was found to increase the number of CD4⁺ T cells, stimulate the secretion of interleukins (IL)-17 and IL-21, and the expression of transcription factor (retinoic acid-related orphan receptor (ROR)-γt). The levels of transforming growth factor (TGF)-β3 and transcription factor (forkhead box (Fox)p-3) were increased in NCSP-treated groups. Moreover, NCSP upregulated the mRNA expression of toll like receptors (TLR-2, -6 and -9), while it downregulated the TLR-4 expression. In addition, NCSP modulated the intestinal microbiota composition and increased the levels of SCFAs. These findings indicated that NCSP may enhance intestinal immunity and have the potential to become a prebiotic to regulate intestinal microbiota.

Conventional CD4+ T cells present bacterial antigens to induce cytotoxic and memory CD8+ T cell responses

Bacterial phagocytosis and antigen cross-presentation to activate CD8⁺ T cells are principal functions of professional antigen presenting cells. However, conventional CD4⁺ T cells also capture and kill bacteria from infected dendritic cells in a process termed transphagocytosis (also known as transinfection). Here, we show that transphagocytic T cells present bacterial antigens to naive CD8⁺ T cells, which proliferate and become cytotoxic in response. CD4⁺ T-cell-mediated antigen presentation also occurs in vivo in the course of infection, and induces the generation of central memory CD8⁺ T cells with low PD-1 expression. Moreover, transphagocytic CD4⁺ T cells induce protective anti-tumour immune responses by priming CD8⁺ T cells, highlighting the potential of CD4⁺ T cells as a tool for cancer immunotherapy.

Antigen presentation is generally considered the domain of innate immune cells, but CD4⁺ T cells can transphagocytose bacteria from infected dendritic cells. Here the authors show CD4⁺ T cells can transphagocytose bacterial and tumour antigens and present them to CD8⁺ T cells to activate memory and cytotoxic functions.

Building conventions for unconventional lymphocytes

Falling between the classical characteristics of innate immune cells and adaptive T and B cells are a group of lymphocytes termed "unconventional." These cells express antigen-specific T or B cell receptors, but behave with innate characteristics. Well-known members of this group include the gamma-delta T cell and the Natural Killer T cell. Recent literature has greatly expanded scientific knowledge of unconventional lymphocytes, but key questions remain unresolved in the field, including why these cells have been maintained concurrently with conventional innate and adaptive immune cells. Here, we summarize current literature that suggests what their unique purposes may be, including specialized functions with the microbiota and in early development. From the consensus literature, we discuss where we see unconventional lymphocytes fit into the logical organization of the complete immune system.