Tuft cells and fibroblasts promote thymus regeneration through ILC2-mediated type 2 immune response.

The thymus is a primary lymphoid organ that is essential for the establishment of adaptive immunity through generation of immunocompetent T cells. In response to various stress signals, the thymus undergoes acute but reversible involution. However, the mechanisms governing its recovery are incompletely understood. Here, we used a dexamethasone-induced acute thymic involution mouse model to investigate how thymic hematopoietic cells (excluding T cells) contribute to thymic regeneration. scRNA-seq analysis revealed marked transcriptional and cellular changes in various thymic populations and highlighted thymus-resident innate lymphoid cells type 2 (ILC2) as a key cell type involved in the response to damage. We identified that ILC2 are activated by the alarmins IL-25 and IL-33 produced in response to tissue damage by thymic tuft cells and fibroblasts, respectively. Moreover, using mouse models deficient in either tuft cells and/or IL-33, we found that these alarmins are required for effective thymus regeneration after dexamethasone-induced damage. We also demonstrate that upon their damage-dependent activation, thymic ILC2 produce several effector molecules linked to tissue regeneration, such as amphiregulin and IL-13, which in turn promote thymic epithelial cell differentiation. Collectively, our study elucidates a previously undescribed role for thymic tuft cells and fibroblasts in thymus regeneration through activation of the type 2 immune response.

Segmented filamentous bacteria-induced epithelial MHCII regulates cognate CD4+ IELs and epithelial turnover.

Intestinal epithelial cells have the capacity to upregulate MHCII molecules in response to certain epithelial-adhesive microbes, such as segmented filamentous bacteria (SFB). However, the mechanism regulating MHCII expression as well as the impact of epithelial MHCII-mediated antigen presentation on T cell responses targeting those microbes remains elusive. Here, we identify the cellular network that regulates MHCII expression on the intestinal epithelium in response to SFB. Since MHCII on the intestinal epithelium is dispensable for SFB-induced Th17 response, we explored other CD4+ T cell-based responses induced by SFB. We found that SFB drive the conversion of cognate CD4+ T cells to granzyme+ CD8α+ intraepithelial lymphocytes. These cells accumulate in small intestinal intraepithelial space in response to SFB. Yet, their accumulation is abrogated by the ablation of MHCII on the intestinal epithelium. Finally, we show that this mechanism is indispensable for the SFB-driven increase in the turnover of epithelial cells in the ileum. This study identifies a previously uncharacterized immune response to SFB, which is dependent on the epithelial MHCII function.

Thymic mimetic cells function beyond self-tolerance.

Development of immunocompetent T cells in the thymus is required for effective defence against all types of pathogens, including viruses, bacteria and fungi. To this end, T cells undergo a very strict educational program in the thymus, during which both non-functional and self-reactive T cell clones are eliminated by means of positive and negative selection1.Thymic epithelial cells (TECs) have an indispensable role in these processes, and previous studies have shown the notable heterogeneity of these cells2-7. Here, using multiomic analysis, we provide further insights into the functional and developmental diversity of TECs in mice, and reveal a detailed atlas of the TEC compartment according to cell transcriptional states and chromatin landscapes. Our analysis highlights unconventional TEC subsets that are similar to functionally well-defined parenchymal populations, including endocrine cells, microfold cells and myocytes. By focusing on the endocrine and microfold TEC populations, we show that endocrine TECs require Insm1 for their development and are crucial to maintaining thymus cellularity in a ghrelin-dependent manner; by contrast, microfold TECs require Spib for their development and are essential for the generation of thymic IgA+ plasma cells. Collectively, our study reveals that medullary TECs have the potential to differentiate into various types of molecularly distinct and functionally defined cells, which not only contribute to the induction of central tolerance, but also regulate the homeostasis of other thymus-resident populations.

Transcriptional regulation of the thymus master regulator Foxn1.

FOXN1 is a transcription factor critical for the development of both thymic epithelial cell (TEC) and hair follicle cell (HFC) compartments. However, mechanisms controlling its expression remain poorly understood. To address this question, we performed thorough analyses of the evolutionary conservation and chromatin status of the Foxn1 locus in different tissues and states and identified several putative cis-regulatory regions unique to TECs versus HFCs. Furthermore, experiments using genetically modified mice with specific deletions in the Foxn1 locus and additional bioinformatic analyses helped us identify key regions and transcription factors involved in either positive or negative regulation of Foxn1 in both TECs and HFCs. Specifically, we identified SIX1 and FOXN1 itself as key factors inducing Foxn1 expression in embryonic and neonatal TECs. Together, our data provide important mechanistic insights into the transcriptional regulation of the Foxn1 gene in TEC versus HFC and highlight the role of FOXN1 in its autoregulation.

The acetyltransferase KAT7 is required for thymic epithelial cell expansion, expression of AIRE target genes, and thymic tolerance.

The autoimmune regulator (AIRE) induces the transcription of thousands of peripheral tissue genes (PTGs) in thymic epithelial cells (TECs) to mediate immunological tolerance. The chromatin state required for optimal AIRE function in TECs and how this state is induced remains unclear. We tested the role of the histone acetyltransferase, KAT7 (also known as HBO1 or MYST2), which is essential for acetylation of histone 3 lysine 14, in TEC differentiation, AIRE-mediated PTG expression, and thymic tolerance. We find that KAT7 is required for optimal expansion of medullary TEC and has a major role in the expression of AIRE-dependent PTGs, associated with enhanced chromatin accessibility at these gene loci in TECs. Mice with TEC-specific Kat7 deletion develop organ-specific autoimmunity with features resembling those observed in Aire-deficient mice. These findings highlight critical roles for KAT7-mediated acetylation in promoting a chromatin state at PTG loci that enables AIRE function and the establishment of immunological tolerance.

Thymic epithelial cell heterogeneity: TEC by TEC.

The generation of a functional T cell repertoire in the thymus is mainly orchestrated by thymic epithelial cells (TECs), which provide developing T cells with cues for their navigation, proliferation, differentiation and survival. The TEC compartment has been segregated historically into two major populations of medullary TECs and cortical TECs, which differ in their anatomical localization, molecular characteristics and functional roles. However, recent studies have shown that TECs are highly heterogeneous and comprise multiple subpopulations with distinct molecular and functional characteristics, including tuft cell-like or corneocyte-like phenotypes. Here, we review the most recent advances in our understanding of TEC heterogeneity from a molecular, functional and developmental perspective. In particular, we highlight the key insights that were recently provided by single-cell genomic technologies and in vivo fate mapping and discuss them in the context of previously published data.

Transcriptional programs that control expression of the autoimmune regulator gene Aire.

Aire is a transcriptional regulator that induces promiscuous expression of thousands of genes encoding tissue-restricted antigens (TRAs) in medullary thymic epithelial cells (mTECs). While the target genes of Aire are well characterized, the transcriptional programs that regulate its own expression have remained elusive. Here we comprehensively analyzed both cis-acting and trans-acting regulatory mechanisms and found that the Aire locus was insulated by the global chromatin organizer CTCF and was hypermethylated in cells and tissues that did not express Aire. In mTECs, however, Aire expression was facilitated by concurrent eviction of CTCF, specific demethylation of exon 2 and the proximal promoter, and the coordinated action of several transcription activators, including Irf4, Irf8, Tbx21, Tcf7 and Ctcfl, which acted on mTEC-specific accessible regions in the Aire locus.

HDAC3 Is a Master Regulator of mTEC Development.

The thymus provides a unique microenvironment enabling development and selection of T lymphocytes. Medullary thymic epithelial cells (mTECs) play a pivotal role in this process by facilitating negative selection of self-reactive thymocytes and the generation of Foxp3(+) regulatory T cells. Although studies have highlighted the non-canonical nuclear factor κB (NF-κB) pathway as the key regulator of mTEC development, comprehensive understanding of the molecular pathways regulating this process still remains incomplete. Here, we demonstrate that the development of functionally competent mTECs is regulated by the histone deacetylase 3 (Hdac3). Although histone deacetylases are global transcriptional regulators, this effect is highly specific only to Hdac3, as neither Hdac1 nor Hdac2 inactivation caused mTEC ablation. Interestingly, Hdac3 induces an mTEC-specific transcriptional program independently of the previously recognized RANK-NFκB signaling pathway. Thus, our findings uncover yet another layer of complexity of TEC lineage divergence and highlight Hdac3 as a major and specific molecular switch crucial for mTEC differentiation.

The deacetylase Sirt1 is an essential regulator of Aire-mediated induction of central immunological tolerance.

Aire is a transcriptional regulator that induces the promiscuous expression of thousands of tissue-restricted antigens (TRAs) in medullary thymic epithelial cells (mTECs), a step critical for the induction of immunological self-tolerance. Studies have offered molecular insights into how Aire operates, but more comprehensive understanding of this process still remains elusive. Here we found abundant expression of the protein deacetylase Sirtuin-1 (Sirt1) in mature Aire(+) mTECs, wherein it was required for the expression of Aire-dependent TRA-encoding genes and the subsequent induction of immunological self-tolerance. Our study elucidates a previously unknown molecular mechanism for Aire-mediated transcriptional regulation and identifies a unique function for Sirt1 in preventing organ-specific autoimmunity.

Fish oil attenuates surgery-induced immunosuppression, limits post-operative metastatic dissemination and increases long-term recurrence-free survival in rodents inoculated with cancer cells.

BACKGROUND & AIMS: Omega-3 fatty acids (ω-3FA) attenuate postoperative immunosuppression vis-à-vis infection. Since immune-surveillance targets metastasizing cancer cells, we assessed the effect of ω-3FA consumption on 1) early post-operative Natural Killer cell (NK) cytotoxicity and metastases and 2) long-term recurrence-free survival, in two rodent models of surgery-promoted metastases. METHODS: C57BL/6J mice were fed standard, ω-3FA-enriched, or ω-6FA-enriched chow, beginning one week before subcutaneous footpad implantation of syngeneic melanoma cells. When tumors reached the volume of 110 µl, the tumor-bearing footpad was amputated, and long-term recurrence-free survival was assessed. Also, F344 rats were fed ω-3FA or ω-6FA for a month before undergoing or not undergoing laparotomy, and were intravenously inoculated with radio-labeled syngeneic adenocarcinoma cells. Marginating-pulmonary (MP)-leukocytes were harvested, and lung tumor retention (LTR) of metastases was assessed. RESULTS: ω-3FA consumption did not affect the growth of footpad tumors, but significantly enhanced post-amputation recurrence-free survival in mice. Surgery had a deleterious effect on NK cell activity and LTR whereas ω-3FA had large beneficial effects in non-operated rats and an even greater impact in operated rats. CONCLUSIONS: ω-3FA feeding attenuates or even overcomes postoperative NK cell suppression, increases resistance to experimental and spontaneous metastasis, and enhances recurrence-free survival following excision of metastasizing primary tumors. These findings warrant clinical studies of ω-3FA-based nutrition in patients undergoing resection of a primary tumor.

Improving postoperative immune status and resistance to cancer metastasis: a combined perioperative approach of immunostimulation and prevention of excessive surgical stress responses.

BACKGROUND: Surgical procedures, including primary tumor resection, have been suggested to suppress immune competence and to promote postoperative infections and cancer metastasis. Catecholamines and prostaglandins were recently implicated in these processes, and in directly promoting tumor angiogenesis and invasion. OBJECTIVE: To examine the integration of 2 complementary approaches to reduce postoperative immunosuppression and metastatic progression: (1) perioperative immunostimulation with CpG-C and (2) pharmacological blockade of the tumor-promoting and immunosuppressing effects of catecholamines and prostaglandins, using propranolol (P) and etodolac (E), respectively. METHODS: F344 rats were treated before surgery with CpG-C, P+E, both interventions, or vehicles, and were intravenously inoculated with syngeneic MADB106 mammary adenocarcinoma cells. Blood was withdrawn, marginating-pulmonary leukocytes were harvested, and NK activity and lung MADB106 tumor retention were assessed. In addition, C57BL/6 mice were implanted with syngeneic B16F10.9 melanoma cells. When tumors reached 100 mm, mice were treated with CpG-C/vehicle, and 24 hours later the tumor was excised along with P+E/vehicle treatment. Recurrence-free survival was monitored thereafter. RESULTS: Each of the regimens alone, CpG-C or P+E, showed improvement in most indices examined, including improved long-term recurrence-free survival rates. Most importantly, the combined treatment yielded additive or synergistic effects, further improving tumor clearance from the lungs and enhancing NK numbers and cytotoxicity via different, but complimentary, mechanisms. CONCLUSIONS: Treatment aimed at perioperative enhancement of CMI and simultaneous inhibition of excessive catecholamine and prostaglandin responses, employing CpG-C, propranolol, and etodolac, could be successful in limiting postoperative immunosuppression and metastatic progression, more so than each treatment alone.

Continuous stress disrupts immunostimulatory effects of IL-12.

Immune stimulation by biological response modifiers is a common approach in tumor immunotherapy. IL-12 was found effective in various animal studies, but clinical trials showed limited success. However, among other differences, animal models do not simulate psychological or physiological stress while employing IL-12, whereas cancer patients often experience distress while treated with immunostimulants. Thus, in the current study we assessed the impact of continuous stress on the efficacy of IL-12 immunostimulation. F344 rats were subjected to a pharmacological stress paradigm (continuous administration of a ß-adrenergic agonist) or to a 20 h behavioral stress paradigm (wet cage exposure) commencing 2h before IL-12 administration. Twenty-six hours after stress initiation, we studied indices known to reflect IL-12 immunostimulatory impacts, including NK cell numbers and activity in different immune compartments, and in vivo resistance to MADB106 lung tumor colonization. The results indicated that both the pharmacological and behavioral stress paradigms significantly reduced the increase in the number and activity of marginating-pulmonary NK cells evident in non-stressed IL-12 treated animals. Additionally, stressed animals exhibited a lower IL-12-induced improvement of MADB106 lung clearance, an in vivo index that markedly depends on total marginating-pulmonary NK activity. These deleterious effects of stress were more prominent in males than in females. Overall, the findings demonstrate that prolonged stress exposure can disrupt the efficacy of simultaneous immunostimulatory treatments, irrespective of stress effects on baseline immune measures. Neuroendocrine and cellular mediating mechanisms are yet unknown, but the potential clinical ramifications of these findings warrant consideration in clinical trials employing immunostimulatory agents.

The marginating-pulmonary immune compartment in rats: characteristics of continuous inflammation and activated NK cells.

A significant role has been indicated for cellular immunity in controlling circulating cancer cells, but most autologous tumor cells seem resistant, in vitro, to natural killer cell (NKC) and cytotoxic T lymphocytes cytotoxicity. Addressing this apparent contradiction, we recently identified a unique leukocyte population, marginating-pulmonary (MP)-leukocytes, which exhibit potent natural killer (NK) cytotoxicity. Here, we characterize the MP-compartment in naive and immunostimulated rats, and assessed its cytotoxicity against "NK-resistant" tumors cells. Animals were treated with poly I-C (3x0.2 mg/kg) or saline, and circulating-leukocytes and MP-leukocytes were collected and analyzed in terms of cellular composition, cellular activation markers, and NK cytotoxicity of leukocytes and purified NKCs. Compared with circulating-leukocytes, MP-leukocytes showed greater proportion of granulocytes, monocytes, NKCs, and large NKCs; higher expression of activation and adhesion markers (CD25, CD11a, CD11b, and NKR-P1, IFN-gamma); and elevated NK cytotoxicity of leukocytes and purified NKCs against several syngeneic and xenogeneic NK-resistant target cells (from both F344 and BDX inbred rats). In immunostimulated animals (treated with poly I-C), but not in naive animals, purified NKCs from the MP-compartment showed markedly superior cytotoxicity, suggesting that poly I-C immunostimulation uniquely affect MP-NKCs, and that in naive animals other MP-leukocytes support NK cytotoxicity. Overall, the results suggest that the MP-compartment is characterized by a continuous activated inflammatory microenvironment uniquely affected by immunostimulation. If similarly potent MP-NKCs exist in patients, then circulating autologous tumor cells that are considered "NK-resistant" could actually be controlled by MP-NKCs. Innate immunity may assume greater role in controlling malignant spread, especially after immunostimulation.

CpG-C oligodeoxynucleotides limit the deleterious effects of beta-adrenoceptor stimulation on NK cytotoxicity and metastatic dissemination.

Suppression of natural killer (NK) cell activity is common after stress, has been reported to predict malignant recurrence in cancer patients, and was shown to underlie metastatic dissemination in animal models. We have previously reported that catecholamines play a major role in NK cell suppression, particularly in the context of physiologic stress and surgery. In the current study using Fisher 344 rats, we examined the prophylactic use of different regimens of type-C CpG oligodeoxynucleotides (CpG-C ODN) on NK activity and metastatic dissemination in the context of pharmacologic stress (using metaproterenol for beta-adrenoceptor stimulation). Our results indicated that the beneficial effects of CpG-C ODN were more profound under pharmacologic stress than under baseline conditions. A bolus of CpG-C ODN (330 microg/kg, intraperitoneally) 24 hours before metaproterenol-challenge was most effective at reducing lung tumor retention of an experimental syngeneic mammary adenocarcinoma (MADB106), although having no observable side effects. Depletion of NK cells revealed their key role in improving baseline levels of resistance to metastatic dissemination after CpG-C ODN administration. When NK cell cytotoxicity was assessed in the circulation and the marginating-pulmonary immune compartments, we found that CpG-C ODN protected individual NK cells from metaproterenol-induced suppression in both compartments. Moreover, in the critical marginating-pulmonary compartment, CpG-C ODN also elevated baseline cytotoxicity per NK cell against MADB106 tumor cells, and increased NK cell numbers in nonstressed rats. Overall, prophylactic CpG-C ODN treatment can improve immunocompetence and potentially reduce metastatic dissemination, especially in clinical settings characterized by enhanced sympathetic stress responses.

Perioperative use of beta-blockers and COX-2 inhibitors may improve immune competence and reduce the risk of tumor metastasis.

BACKGROUND: COX inhibitors and beta-blockers were recently suggested to reduce cancer progression through inhibition of tumor proliferation and growth factor secretion, induction of tumor apoptosis, and prevention of cellular immune suppression during the critical perioperative period. Here we evaluated the perioperative impact of clinically applicable drugs from these categories in the context of surgery, studying natural killer (NK) cell activity and resistance to experimental metastases. METHODS: F344 rats were treated with COX-1 inhibitors (SC560), COX-2 inhibitors (indomethacin, etodolac, or celecoxib), a beta-blocker (propranolol), or a combination of a COX-2 inhibitor and a beta-blocker (etodolac and propranolol). Rats underwent laparotomy, and were inoculated intravenously with syngeneic MADB106 tumor cells for the assessment of lung tumor retention (LTR). Additionally, the impact of these drug regimens on postoperative levels of NK cytotoxicity was studied in peripheral blood and marginating-pulmonary leukocytes. RESULTS: Surgery increased MADB106 LTR. COX-2 inhibition, but not COX-1 inhibition, reduced postoperative LTR. Etodolac and propranolol both attenuated the deleterious impact of surgery, and their combined use abolished it. Surgery decreased NK cytotoxicity per NK cell in both immune compartments, and only the combination of etodolac and propranolol significantly attenuated these effects. Lastly, the initiation of drug treatment three days prior to surgery yielded the same beneficial effects as a single pre-operative administration, but, as discussed, prolonged treatment may be more advantageous clinically. CONCLUSIONS: Excess prostaglandin and catecholamine release contributes to postoperative immune-suppression. Treatment combining perioperative COX-2 inhibition and beta-blockade is practical in operated cancer patients, and our study suggests potential immunological and clinical benefits.

Immune suppression while awaiting surgery and following it: dissociations between plasma cytokine levels, their induced production, and NK cell cytotoxicity.

Surgery may render patients susceptible to life-threatening complications, including infections and later metastases. Suppression of cell mediated immunity (CMI) and perturbations in the cytokine network were implicated in these outcomes. The current study assessed the effects of various surgeries on a wide array of immune indices, and compared patients' pre-operative immune status to that of control subjects. A total of 81 subjects (controls, moderate and major surgeries) provided up to five daily blood samples. Whole blood procedures were conducted within hours of blood withdrawal, assessing NK cell number and cytotoxicity, and plasma cytokine levels and induced production (IFNgamma, IL-6, IL-10, and IL-12). Our findings indicate that surgery reduced NK cell numbers/ml blood, and independently suppressed NK activity per NK cell and per ml blood. Among other perturbations in the cytokine network, pro-CMI cytokine production (IL-12 and IFNgamma) was reduced by surgery. Surprisingly, plasma levels of IFNgamma and IL-6 increased following surgery, while their in vitro induced production showed opposite effects. Patients awaiting surgery exhibited impaired IL-12 induced production and NK activity/ml, and reduced IFNgamma plasma levels. No significant associations were found between NK cytotoxicity and Th1 cytokines, although these indices showed high correlations with other variables. Overall, our findings indicate that patients exhibit impaired immune functions even before operation, which seem to contribute to the evident post-operative immune suppression. In the peri-operative context, induced cytokine production and plasma cytokines levels reflect different processes. Last, we suggest that peri-operative suppression of NK activity is mediated by neuroendocrine responses rather than Th1 cytokines.

Orphanin FQ/nociceptin interactions with the immune system in vivo: gene expression changes in lymphoid organs and regulation of the cytokine response to staphylococcal enterotoxin A.

Orphanin FQ/nociceptin (OFQ/N) is a neuropeptide implicated in immunomodulation. Here, we show that endogenous and exogenous OFQ/N modulated the cytokine response to the bacterial superantigen staphylococcal enterotoxin A (SEA). Specifically, OFQ/N enhanced TNFalpha and IFNgamma transcripts in the spleen when injected prior to SEA challenge. Moreover, mice lacking the OFQ/N precursor gene showed diminished TNFalpha and IFNgamma mRNA induction in the spleen following SEA challenge. In addition, mRNA levels of ppOFQ/N and the OFQ/N receptor, NOP, were altered in thymus and spleen after SEA challenge. Overall, this suggests that the OFQ/N system can influence immune function and is itself influenced by immune stimuli.

Neuronal, endocrine, and anorexic responses to the T-cell superantigen staphylococcal enterotoxin A: dependence on tumor necrosis factor-alpha.

Staphylococcal enterotoxin A (SEA) is a microbial superantigen that activates T-lymphocytes and induces production of various cytokines, including tumor necrosis factor-alpha (TNFalpha). Previously, it was shown that SEA activates the hypothalamic-pituitary-adrenal axis and augments gustatory neophobic behaviors. In the present study, it was hypothesized that these effects involve neuronal activation in forebrain regions mediating fear and/or anxiety and are dependent on the production of TNFalpha. Male C57BL/6J mice were given intraperitoneal injections of 10 microg of SEA and 5 microg of lipopolysaccharide (LPS) or saline and perfused 2 h later for histochemical determination of brain c-Fos immunoreactivity (IR). The results showed increased c-Fos IR in the paraventricular nucleus, arcuate nucleus, central nucleus of the amygdala, bed nucleus of the stria terminalis, and lateral septum. Challenge of TNF-/- mice with SEA did not produce a significant increase in brain c-Fos IR, although c-Fos was increased after exposure to a psychogenic stressor (i.e., open field). In additional experiments, the elevated corticosterone response to SEA was abrogated in TNF-/- mice and was shown to be corticotropin-releasing hormone dependent. Finally, the augmented reduction in novel food intake after SEA challenge was attenuated in TNF-/- mice as well as in wild-type mice administered antibody to TNFalpha. In conclusion, challenge with SEA recruits brain regions mediating stress and anxiety responses, an effect that requires endogenous TNFalpha. Whether this is indicative of all T-cell superantigens remains to be determined, although it stands in contrast to other models of neuroimmunomodulation (e.g., LPS) that involve multiple cytokine influences.

Neural and behavioral responses to systemic immunologic stimuli: a consideration of bacterial T cell superantigens.

Immune responses represent a source of systemic stress which impacts the brain and modifies various neuroendocrine and behavioral functions. Therefore, the immune system has been conceived of as a potential contributor to stress-related behavioral abnormalities, such as depression. Much of this knowledge has been gained through research focused largely on the administration of cytokines and/or bacterial endotoxin (eg., LPS), which targets innate immune cells, such as macrophages. However, fewer studies have addressed the effects of T cell activation on central nervous system (CNS) function. The discovery and characterization of bacterial superantigens (SAgs) has introduced an important opportunity for studying how T cell activation influences CNS function. Superantigens target unique variable regions of the beta chain of the mouse and human T cell receptor. This is restricted by the class II molecule of the major histocompatibility complex (MHC), and results in the production of a cytokine cascade that includes interleukin-2 (IL-2), interferon-gamma (IFNgamma), tumor necrosis factor (TNF) and many other cytokines, including IL-6. The best studied SAgs are the staphylococcal enterotoxins, of which staphylococcal enterotoxins A and B (SEA and SEB), have been shown to produce significant changes in behavior and activation of the hypothalamic-pituitary-adrenal (HPA) axis. Importantly, a T cell requirement was necessary to produce these changes. Furthermore, the anorexic or hypophagic effects of SAg challenge in mice appears to be related to anxiety-like processes, since challenge with both SEA or SEB reduces consumption of mainly novel food or food presented in a novel context. In the present paper, these studies are reviewed and related to known alterations in both anxiogenic and anxiolytic neuropeptides. It is suggested that immunologically-induced changes in the brain activate both categories of neuropeptides, thereby sustaining an adaptive state of arousal that promotes appropriate behavioral adjustments during infectious illness.