Lifelong Maintenance of Oral Tolerance and Immunity Profiles in Mice Depends on Early Exposure to Antigen.

Oral tolerance is defined as a state of systemic hyporesponsiveness to an antigen that has been previously administered by the oral route. Many factors affect oral tolerance induction; some of them related to antigen, and some related to the animal. The age of the animal is one of the most important factors that affect oral tolerance as ageing brings many alterations in immune responses. Herein, we demonstrated that both the oral tolerance and pattern of immune reactivity triggered in early life were kept up to 15 months regarding the magnitude of antibody production, cell proliferation and cytokine profile when compared to immune responses induced in old mice. Therefore, our results corroborate with a promising proposal for prevaccination during childhood and young age, and a booster in older age, to make sure that the primary immunization in early life is not lost in aged individuals.

Comparative phenotypic profile of subpopulations of peripheral blood leukocytes in European (Bos taurus taurus) and Zebu cattle (Bos taurus indicus).

Differences in cellular and humoral immunity in Zebu (Bos taurus indicus) and European (B. taurus taurus) cattle breeds, which may be related to differences in resistance or susceptibility to infectious or parasitic diseases, are largely unknown. This study aimed to perform a comparative analysis of innate and adaptive immunity of European (including Holstein, Brown Swiss, and Hereford) and Zebu (including Gir, Nelore, and Guzera) breeds, by assessing their peripheral blood leukocyte profiles (i.e., monocytes, eosinophils, and lymphocytes, including CD4(+) and CD8(+) T cells, and CD21(+) B cells). Higher frequencies of cells involved in innate immunity were observed in Zebu breeds, particularly monocytes and non-T and non-B cells (13.37 ± 0.9058 and 37.67 ± 1.55, respectively). This finding may contribute to the increased resistance of B. taurus indicus to certain infectious and parasitic diseases. Considering other leukocyte populations in the peripheral blood, among-breed variation was greater than differences between the two subspecies. This study will serve as a basis for further investigations regarding comparative immunology and resistance to infectious and parasitic diseases of cattle.

Living in endemic area for infectious diseases accelerates epigenetic age.

Inflammaging is a low-grade inflammatory state generated by the aging process that can contribute to frailty and age-related diseases in the elderly. However, it can have distinct effects in the elderly living in endemic areas for infectious diseases. An increased inflammatory response may confer protection against infectious agents in these areas, although this advantage can cause accelerating epigenetic aging. In this study, we evaluated the inflammatory profile and the epigenetic age of infected and noninfected individuals from an endemic area in Brazil. The profile of cytokines, chemokines and growth factors analyzed in the sera of the two groups of individuals showed similarities, although infected individuals had a higher concentration of these mediators. A significant increase in IL-1ra, CXCL8, CCL2, CCL3 and CCL4 production was associated with leprosy infection. Notably, elderly individuals displayed distinct immune responses associated with their infection status when compared to adults suggesting an adaptive remodelling of their immune responses. Epigenetic analysis also showed that there was no difference in epigenetic age between the two groups of individuals. However, individuals from the endemic area had a significant accelerated aging when compared to individuals from São Paulo, a non-endemic area in Brazil. Moreover, the latter cohort was also epigenetically aged in relation to an Italian cohort. Our data shows that living in endemic areas for chronic infectious diseases results in remodelling of inflammaging and acceleration of epigenetic aging in individuals regardless of their infectious status. It also highlights that geographical, genetic and environmental factors influence aging and immunosenescence in their pace and profile.

Splenectomy increases mortality in murine Trypanosoma cruzi infection.

The spleen is a secondary lymphoid organ that harbours a variety of cells such as T and B lymphocytes and antigen-presenting cells important to immune response development. In this study, we evaluated the impact of spleen removal in the immune response to experimental Trypanosoma cruzi infection. C57BL/6 mice were infected with Y strain of the parasite and infection was followed daily. Mice that underwent splenectomy had fewer parasites in peripheral blood at the peak of infection; however, mortality was increased. Histological analysis of heart and liver tissues revealed an increased number of parasites and inflammatory infiltrates at these sites. Spleen removal was associated with reduction in IFN-γ and TNF-α production during infection as well as with a decrease in specific antibody secretion. Haematological disorders were also detected. Splenectomized mice exhibited severe anaemia and decreased bone marrow cell numbers. Our results indicate that spleen integrity is critical in T. cruzi infection for the immune response against the parasite, as well as for the control of bone marrow haematological function.

Ageing down-modulates liver inflammatory immune responses to schistosome infection in mice.

Ageing is associated with several alterations in the immune system. Our aim in this study was to compare the development of immunity to Schistosoma mansoni infection in young versus aged C57Bl/6 mice using the liver as the main organ to evaluate pathological alterations and immune responses. In the acute phase, young mice had large liver granulomas with fibrosis and inflammatory cells. Chronic phase in young animals was associated with immunomodulation of granulomas that became reduced in size and cellular infiltrate. On the other hand, aged animals presented granulomas of smaller sizes already in the acute phase. Chronic infection in these mice was followed by no alteration in any of the inflammatory parameters in the liver. In concert with this finding, there was an increase in activated CD4+ T, CD19+ B and NK liver cells in young mice after infection whereas old mice had already higher frequencies of activated B, NK and CD4+ T liver cells and infection does not change these frequencies. After infection, liver production of inflammatory and regulatory cytokines such as IFN-gamma, IL-4 and IL-10 increased in young but not in old mice that had high levels of IL-4 and IL-10 regardless of their infection status. Our data suggest that the unspecific activation status of the immune system in aged mice impairs inflammatory as well as regulatory immune responses to S. mansoni infection in the liver, where major pathological alterations and immunity are at stage. This poor immune reactivity may have a beneficial impact on disease development.

Effect of Saccharomyces cerevisiae UFMG A-905 in a murine model of food allergy.

Food allergy is triggered when there is an abnormal activation of the immune system by food allergens. Currently, there is no curative therapy for this pathological condition. Due to the immunomodulatory properties of probiotics they are potential candidates as therapeutic tools for food allergy. Therefore, the aim of this study was to evaluate the probiotic effect of Saccharomyces cerevisiae UFMG A-905 (905) in an in vivo model of food allergy. Probiotic effect was assessed by clinical, histological, immunological and microbiological parameters analysis. Furthermore, we also evaluated if 905 after inactivation has an effect, as well as if such an effect is dose dependent. Our results showed that oral administration of only viable 905 promotes a significant attenuation of tissue injury and myeloperoxidase (MPO) activity levels. Moreover, the treatment reduced interleukin 17 levels, and administration of the supernatant from the yeast culture also promoted a significant decrease in MPO levels. However, considering the systemic parameters, immunoglobulin (Ig)E and IgG anti-ovalbumin, which are essentials for triggering the allergic process, there was no effect, suggesting that the yeast promotes a local but not a systemic effect in the model evaluated. In addition, we found that only high doses of viable 905 were able to attenuate the signs of inflammation. In conclusion, oral administration of 905 led to a local effect that depends on the viability of the yeast.

Specific immune responses but not basal functions of B and T cells are impaired in aged mice.

Senescence is characterized by several alterations in the immune system. Such modifications can be found in lymphoid organs as well as in the cellular components of the immune system. Several reports have suggested that immune dysfunction can affect both T and B cells, but T cells have been shown to be more susceptible to the effects of aging. B cell function may also be altered with reduction in germinal center formation, antibody response, and affinity maturation of antibodies. Herein we showed that although antigen-specific antibody response to a soluble antigen declines in 18-month old mice, total levels of serum antibodies as well as frequencies of spleen and bone marrow antibody-producing cells are increased in aged mice. In addition, proliferative response of non-stimulated spleen T cells from aged mice were augmented and insensitive to increasing doses of concanavalin A stimulation as compared to young mice that showed a typical dose-dependent response to mitogen stimulation in vitro. These data suggest that the higher activation mode of B and T cells in senescent mice is a result of an increased frequency of cells committed to previous antigenic experiences and with poor ability to respond to novel antigenic challenges.

Short-term administration of ethanol in mice deviates antigen presentation activity towards B cells.

Alcohol has a variety of short- and long-term effects on cell-mediated and humoral immune response. Herein, we have characterized the impact of high-dose EtOH administration on phenotypic and functional features of murine APC subsets, including dendritic cell (DC), macrophages and B cells. Impaired cytokine synthesis and Leishmania-phagocytosis was observed in peritoneal macrophages following EtOH administration. Moreover, EtOH exposure led to decreased levels of splenic myeloid DC and increased percentage of macrophages with no changes in splenic lymphoid DC and B cells. Adverse effects of short-term EtOH administration also resulted in impaired OVA-endocytosis by DC and macrophages. In contrast, EtOH consumption upregulates OVA-internalization by B cells. These changes on APC hierarchy may play a role shifting the fate of the immune response after EtOH ingestion. In addition to an overall downregulation of Toll-like receptor-TLR-4 expression by splenic APC, a downregulation of TLR-2 expression in macrophages was observed. Moreover, EtOH exposure altered the expression of co-signalling molecules on splenic APC, downregulating CD40 on macrophages and upregulating CD80 on B cells, with no impact on DC subsets. The net result of changes in TLR-mediated and co-stimulatory signals may determine the altered immunological status induced by acute consumption of alcohol. A direct impact of high-dose EtOH administration in the activation status of splenic CD4(+) T cells was observed. Together, our results demonstrated that short-term high-dose EtOH administration has differential impact on APC populations, downregulating splenic macrophages and DC activity but up-regulating B lymphocyte function as APC, and ultimately yielding a micro-environment that led to increased activation of CD4(+) T cells.

CLA-supplemented diet accelerates experimental colorectal cancer by inducing TGF-ß-producing macrophages and T cells.

Conjugated linoleic acid (CLA) has been shown to activate the nuclear receptor PPAR-γ and modulate metabolic and immune functions. Despite the worldwide use of CLA dietary supplementation, strong scientific evidence for its proposed beneficial actions are missing. We found that CLA-supplemented diet reduced mucosal damage and inflammatory infiltrate in the dextran sodium sulfate (DSS)-induced colitis model. Conditional deletion of PPAR-γ in macrophages from mice supplemented with CLA diet resulted in loss of this protective effect of CLA, suggesting a PPAR-γ-dependent mechanism mediated by macrophages. However, CLA supplementation significantly worsened colorectal tumor formation induced by azoxymethane and DSS by inducing macrophage and T-cell-producing TGF-ß via PPAR-γ activation. Accordingly, either macrophage-specific deletion of PPAR-γ or in vivo neutralization of latency-associated peptide (LAP, a membrane-bound TGF-ß)-expressing cells abrogated the protumorigenic effect of CLA. Thus, the anti-inflammatory properties of CLA are associated with prevention of colitis but also with development of colorectal cancer.

Ovalbumin encapsulation into liposomes results in distinct degrees of oral immunization in mice.

Oral administration of protein antigens, such as ovalbumin, may result in induction of either tolerance or immunization. To avoid oral tolerance, there are new strategies to protect the antigens from degradation within the gastrointestinal tract and to allow them to reach inductive immunological sites. One such strategy is the usage of liposomes. Different parameters may influence the stability of liposomes in the gastrointestinal tract. Herein, we studied the immunological consequences of oral administration of liposome-encapsulated ovalbumin in different strains of mice using different liposomes. Our data demonstrated that ovalbumin liposomes improved the induction of oral immunization and the degree of improvement depended on the liposome type and on the strain of mice used. The mechanism responsible for this differential effect of liposomes depended on the site of antigen release and absorption. Therefore, some liposomes might be suitable as adjuvants for oral immunization, others for oral tolerance induction.

Role of mesenteric lymph nodes and aging in secretory IgA production in mice.

Although it is known that Peyer's patches are the major inductive site for S-IgA production and B1 cells contribute to half of the IgA plasma cells detected in the gut lamina propria, the type of contribution of mesenteric lymph nodes to the process is still unclear. Cytokines such as TGF-beta, IL-10, IL-4, IL-5, and IL-6, are required to promote IgA class switching and IgA synthesis. Aging-related alterations in T and B cells and in cytokine production are already known. Some reports have also proposed that S-IgA production might be altered in aged animals. Herein, we investigated the role of MLN and aging in S-IgA production. Two- to 18-month-old BALB/c mice were used to evaluate aging-related alterations and MLN were removed to study its role in S-IgA production. We found that MLN are important, although not essential for S-IgA production. In addition, we showed that production of IgA-related cytokines are well preserved in MLN but not in PP of aged mice and that S-IgA levels are not affected by aging. Our results suggest that MLN may play a complementary role in S-IgA production mostly in aged animals.

The long-term impaired macrophages functions are already observed early after high-dose ethanol administration.

Herein, we described an experimental model of high-dose ethanol (EtOH) administration, able to induce in vitro impairment in macrophage phagocytic capacity, already observed at 24 h after the last EtOH administration. This phenomenon was characterized by enlarged time required for adhesion and internalization events. Parallel studies documented an overall impaired production of interleukin (IL)-6 and nitric oxide (NO) production by peritoneal macrophages in EtOH-treated mice following interferon (IFN)-gamma and lipopolysaccharide (LPS) stimuli. Although the impaired IL-6 response could not be restored by any of the experimental conditions tested, the lower NO response to INF-gamma and LPS was overturned by simultaneous IFN-gamma/LPS stimuli. It was interesting to notice that high-dose EtOH administration drives peritoneal macrophages towards long-term impairment in phagocytosis capacity with slower adhesion time, but with no impact on the time required for internalization. Moreover, 30 days after the last EtOH administration, lower IL-6 response to INF-gamma and impaired NO production were still observed in response to IFN-gamma/LPS stimuli, with the IL-6 response to IFN-gamma being restored by IFN-gamma/LPS stimuli. Histological studies showed that high-dose EtOH administration led to long-term in vivo impairment of antigen-clearance following OVA-driven delayed-type-hypersensitivity induction, characterized by the presence of a large amount of unprocessed OVA surrounded by dermal inflammatory infiltrate, suggesting defective activity of antigen-presenting cells. Together, these findings supported our hypothesis that the poor antigen clearance in vivo may be related to the impaired macrophage function in vitro. These observations in the murine experimental model may reflect some of the consequences of EtOH consumption by humans.

Oral administration of Simbioflora® (synbiotic) attenuates intestinal damage in a mouse model of 5-fluorouracil-induced mucositis.

The use of probiotics to prevent or treat mucosal inflammation has been studied; however, the combined effect of probiotics and prebiotics is unclear. The aim of this study was to test whether oral administration of a synbiotic (Simbioflora®) preparation containing Lactobacillus paracasei, Lactobacillus rhamnosus, Lactobacillus acidophilus and Bifidobacterium lactis plus fructooligosaccharide could help control mucosal inflammation in experimental mucositis induced by 5-fluorouracil (5-FU). Male BALB/c mice were randomly divided into six groups: control (CTL), control + prebiotic (CTL+P), control + synbiotic (CTL+S), mucositis (MUC), mucositis + prebiotic (MUC+P), and mucositis + synbiotic (MUC+S). Mice from the CTL+S, MUC+S, CTL+P, and MUC+P groups received synbiotic or prebiotic daily by oral gavage for 13 days. Mice in the CTL and MUC groups received the same volume of saline. On day 11, mice in the MUC, MUC+P, and MUC+S groups received an intraperitoneal injection of 300 mg/kg 5-FU to induce mucositis. After 72 h, all mice were euthanised. Intestinal permeability, intestinal histology, and biochemical parameters were analysed. Group MUC showed a greater weight loss and increased intestinal permeability (0.020 counts per min [cpm]/g) compared to the CTL group (0.01 cpm/g) P<0.05. Both treatments attenuated weight loss compared to the MUC group. Nonetheless, the synbiotic caused a greater reduction in intestinal permeability (0.012 cpm/g) compared to the MUC (0.020 cpm/g) and MUC+P (0.016 cpm/g) groups P<0.05. Mice in groups MUC+P and MUC+S displayed significant recovery of lesions and maintenance of the mucus layer. There were no differences in the short-chain fatty acid concentrations in the faeces between the MUC and CTL groups (P>0.05). Increased acetate and propionate concentrations were evidenced in the faeces of the MUC+P and MUC+S groups. Only the synbiotic treatment increased the butyrate concentration (P<0.05). The results indicate that administration of synbiotic can decrease mucosal damage caused by mucositis.

The cytosolic sensor STING is required for intestinal homeostasis and control of inflammation.

STING (stimulator of interferon genes) is a cytosolic sensor for cyclic dinucleotides and also an adaptor molecule for intracellular DNA receptors. Although STING has important functions in the host defense against pathogens and in autoimmune diseases, its physiological relevance in intestinal homeostasis is largely unknown. In this study, we show that STING-/- mice presented defective protective mechanisms of intestinal mucosa, including decreased number of goblet cells, diminished mucus production, and lower levels of secretory IgA, when compared with wild-type (WT) mice. Fecal content and microbiota DNA could activate STING, indicating a role of this molecule in gut. Microbiota composition was altered in STING-/- mice toward a more inflammatory profile, evidencing a reduction in the Allobacolum and Bifidobacterium groups along with increase in Disulfovibrio bacteria. Absence of STING lead to decrease in induced intraepithelial lymphocytes (IEL) and to increase in group 1 innate lymphoid cell (ILC1) as well as ILC3 frequencies and decrease in ILC2 in the colon. Development and function of Foxp3+ and LAP+ regulatory T cells were also compromised in STING-/- mice. Moreover, these mice were highly susceptible to dextran sodium sulfate-induced colitis, T-cell-induced colitis, and enteric Salmonella typhimurium infection when compared with WT animals. Therefore, our results identify an important role of STING in maintaining gut homeostasis and also a protective effect in controlling gut inflammation.

Splenectomy does not interfere with immune response to Leishmania major infection in mice.

Spleen is one of the largest lymphoid organs in the body; it harbors immune cells including antigen presenting cells, B and T lymphocytes. It has an important role in humoral and cellular immune responses. Herein we investigated the role of spleen in the immune response to experimental Leishmania major infection. It is known that C57BL/6 mice are resistant to L. major infection whereas BALB/c mice are susceptible. Although splenectomy was associated with reduced serum levels of IFN-gamma, absence of the spleen did not change the profile of L. major infection in the resistant C57BL/6 and BALB/c susceptible mice. Both strains of mice maintained the same profile of cytokine production in regional lymph nodes after splenectomy and responded in the same way against the infection. Only splenectomized BALB/c mice had a reduction in IL-4 and IL-10 production by lymph node cells early in infection. Our data suggest that, in localized infections, regional lymph nodes may replace efficiently the immunological role of spleen in the cellular and humoral immune responses.

Immunoglobulin production is impaired in protein-deprived mice and can be restored by dietary protein supplementation.

Most contacts with food protein and microbiota antigens occur at the level of the gut mucosa. In animal models where this natural stimulation is absent, such as germ-free and antigen-free mice, the gut-associated lymphoid tissue (GALT) and systemic immunological activities are underdeveloped. We have shown that food proteins play a critical role in the full development of the immune system. C57BL/6 mice weaned to a diet in which intact proteins are replaced by equivalent amounts of amino acids (Aa diet) have a poorly developed GALT as well as low levels of serum immunoglobulins (total Ig, IgG, and IgA, but not IgM). In the present study, we evaluated whether the introduction of a protein-containing diet in 10 adult Aa-fed C57BL/6 mice could restore their immunoglobulin levels and whether this recovery was dependent on the amount of dietary protein. After the introduction of a casein-containing diet, Aa-fed mice presented a fast recovery (after 7 days) of secretory IgA (from 0.33 to 0.75 mg/mL, while in casein-fed mice this value was 0.81 mg/mL) and serum immunoglobulin levels (from 5.39 to 10.25 mg/mL of total Ig). Five percent dietary casein was enough to promote the restoration of secretory IgA and serum immunoglobulin levels to a normal range after 30 days feeding casein diet (as in casein-fed mice--15% by weight of diet). These data suggest that the defect detected in the immunoglobulin levels was a reversible result of the absence of food proteins as an antigenic stimulus. They also indicate that the deleterious consequences of malnutrition at an early age for some immune functions may be restored by therapeutic intervention later in life.

Bioactive glass as a drug delivery system of tetracycline and tetracycline associated with beta-cyclodextrin.

The aim of this study was to evaluate the physical-chemical properties, in vivo biocompatibility and antimicrobial activity of bioactive glasses (BG) used as a controlled release device for tetracycline hydrochloride and an inclusion complex formed by tetracycline and beta-cyclodextrin at 1:1 molar ratio. The BG as well as their compounds loaded with tetracycline (BT) and tetracycline:beta-cyclodextrin (BTC) were characterized by FTIR spectroscopy, X-ray powder diffraction, differential scanning calorimetry and by scanning electron microscopy and energy dispersive spectroscopy. The in vivo test was carried out with female mice split into three groups treated with bioactive glass either without drugs, or associated with tetracycline, or with tetracycline:beta-cyclodextrin by subcutaneous implantation. The histological examination of tissue at the site of implantation showed moderate inflammatory reactions in all groups after 72 h. The bacterial effect was tested on A. actinomycetemcomitans suspended in BHI broth, with or without bioactive particles. A considerable bacteriostatic activity was found with BT and BTC glasses, as compared to plain glass. The presence of cyclodextrin was important to slow down the release of tetracycline for a long period of time and it was verified that the presence of tetracycline or its inclusion complex, tetracycline:beta-cyclodextrin, did not affect the bioactivity of the glass.

Association complexes between ovalbumin and cyclodextrins have no effect on the immunological properties of ovalbumin.

Delivery systems are designed to deliver necessary amounts of drugs without modifying their biological features. Cyclodextrins (CD) are potential candidates for such a role due to their ability to alter physical and chemical properties of guest molecules by the formation of inclusion/association complexes. They have already been used to stabilize and solubilize peptides and proteins of biological importance. However, no systematic study has been reported on their immunological effects upon coupling to such proteins. Herein, we prepared and characterized the association complexes between alpha-, beta- and hydroxypropyl-beta-cyclodextrin and ovalbumin (Ova). Afterward we tested the effect of CD coupling in the Ova antigenicity and the immunological effects of CD coupling on Ova oral and subcutaneous administration. Our results clearly show that CD-coupled Ova elicits the same immunological activities as uncoupled Ova. Therefore, we conclude that CDs are immunologically safe for use as delivery systems in animals.

Ethanol-induced colitis prevents oral tolerance induction in mice.

The gut mucosa is a major site of contact with antigens from food and microbiota. Usually, these daily contacts with natural antigens do not result in inflammatory reactions; instead they result in a state of systemic hyporesponsiveness named oral tolerance. Inflammatory bowel diseases (IBD) are associated with the breakdown of the immunoregulatory mechanisms that maintain oral tolerance. Several animal models of IBD/colitis are available. In mice, these include targeted disruptions of the genes encoding cytokines, T cell subsets or signaling proteins. Colitis can also be induced by intrarectal administration of chemical substances such as 2,4,6-trinitrobenzene sulfonic acid in 50% ethanol. We report here a novel model of colitis induced by intrarectal administration of 50% ethanol alone. Ethanol-treated mice develop an inflammatory reaction in the colon characterized by an intense inflammatory infiltrate in the mucosa and submucosa of the large intestine. They also present up-regulation of both interferon gamma (IFN-gamma) and interleukin-4 (IL-4) production by cecal lymph node and splenic cells. These results suggest a mixed type of inflammation as the substrate of the colitis. Interestingly, cells from mesenteric lymph nodes of ethanol-treated mice present an increase in IFN-gamma production and a decrease in IL-4 production indicating that the cytokine balance is altered throughout the gut mucosa. Moreover, induction of oral tolerance to ovalbumin is abolished in these animals, strongly suggesting that ethanol-induced colitis interferes with immunoregulatory mechanisms in the intestinal mucosa. This novel model of colitis resembles human IBD. It is easy to reproduce and may help us to understand the mechanisms involved in IBD pathogenesis.

The conservative physiology of the immune system.

Current immunological opinion disdains the necessity to define global interconnections between lymphocytes and regards natural autoantibodies and autoreactive T cells as intrinsically pathogenic. Immunological theories address the recognition of foreignness by independent clones of lymphocytes, not the relations among lymphocytes or between lymphocytes and the organism. However, although extremely variable in cellular/molecular composition, the immune system preserves as invariant a set of essential relations among its components and constantly enacts contacts with the organism of which it is a component. These invariant relations are reflected, for example, in the life-long stability of profiles of reactivity of immunoglobulins formed by normal organisms (natural antibodies). Oral contacts with dietary proteins and the intestinal microbiota also result in steady states that lack the progressive quality of secondary-type reactivity. Autoreactivity (natural autoantibody and autoreactive T cell formation) is also stable and lacks the progressive quality of clonal expansion. Specific immune responses, currently regarded as the fundament of the operation of the immune system, may actually result from transient interruptions in this stable connectivity among lymphocytes. More permanent deficits in interconnectivity result in oligoclonal expansions of T lymphocytes, as seen in Omenn's syndrome and in the experimental transplantation of a suboptimal diversity of syngeneic T cells to immunodeficient hosts, which also have pathogenic consequences. Contrary to theories that forbid autoreactivity as potentially pathogenic, the physiology of the immune system is conservative and autoreactive. Pathology derives from failures of these conservative mechanisms.