Daniel A. Waizman

Specialized immune cells that reside in tissues orchestrate diverse biological functions by communicating with parenchymal cells¹. The contribution of the innate immune compartment in the meninges and the central nervous system (CNS) is well-characterized; however, whether cells of the adaptive immune system reside in the brain and are involved in maintaining homeostasis is unclear^2,3,4. Here we show that the subfornical organ (SFO) of the brain is a nucleus for parenchymal αβ T cells in the steady-state brain in both mice and humans. Using unbiased transcriptomics, we show that these extravascular T cells in the brain are distinct from meningeal T cells: they secrete IFNγ robustly and express tissue-residence proteins such as CXCR6, which are required for their retention in the brain and for normal adaptive behaviour. These T cells are primed in the periphery by the microbiome, and traffic from the white adipose and gastrointestinal tissues to the brain. Once established, their numbers can be modulated by alterations to either the gut microbiota or the composition of adipose tissue. In summary, we find that CD4 T cells reside in the brain at steady state and are anatomically concentrated in the SFO in mice and humans; that they are transcriptionally and functionally distinct from meningeal T cells; and that they secrete IFNγ to maintain CNS homeostasis through homeostatic fat–brain and gut–brain axes.

Our current understanding of immunity to pathogens suggests that anatomic coupling of antigens with danger signals is a required feature for the formation of immune memory. However, in the context of pathogen-independent inflammation, the stringency of this anatomical coupling is unclear. Here, we demonstrate that multiple modes of skin injury were sufficient to induce a humoral response to antigens introduced in the gut. Skin damage induced a narrow subset of endocrine cytokines that were necessary and sufficient for the priming of antigens introduced at various distal tissues. Thus, in addition to “local priming” of antigen entering through damaged skin, there also exists another paradigm of “remote priming” where anatomical coupling is not essential because of the dissemination of damage-associated intermediaries. Our findings have implications for understanding the fundamental mechanisms of the formation of humoral memory with wide implications for diseases such as food allergy and in vaccinology.

Immunoglobulin A (IgA) is the dominant antibody isotype in the gut and has been shown to regulate microbiota. Mucosal IgA is also widely believed to prevent food allergens from penetrating the gut lining. Even though recent work has elucidated how bacteria-reactive IgA is induced, little is known about how IgA to food antigens is regulated. Although IgA is presumed to be induced in a healthy gut at steady state via dietary exposure, our data do not support this premise. We found that daily food exposure only induced low-level, cross-reactive IgA in a minority of mice. In contrast, induction of significant levels of peanut-specific IgA strictly required a mucosal adjuvant. Although induction of peanut-specific IgA required T cells and CD40L, it was T follicular helper (T_FH) cell, germinal center, and T follicular regulatory (T_FR) cell–independent. In contrast, IgG1 and IgE production to peanut required T_FH cells. These data suggest an alternative paradigm in which the cellular mechanism of IgA production to food antigens is distinct from IgE and IgG1. We developed an equivalent assay to study this process in stool samples from healthy, nonallergic humans, which revealed substantial levels of peanut-specific IgA that were stable over time. Similar to mice, patients with loss of CD40L function had impaired titers of gut peanut-specific IgA. This work challenges two widely believed but untested paradigms about antibody production to dietary antigens: (i) the steady state/tolerogenic response to food antigens includes IgA production and (ii) T_FH cells drive food-specific gut IgA.

Neutrophils are the first immune cells that enter the skin and cause itch in atopic dermatitis.