Dissociation of immune capacity from nutritional status by triiodothyronine supplements in severe protein deficiency

The Tolerance Model of Non-Inflammatory Immune Competence in Acute Pediatric Malnutrition: Origins, Evidence, Test of Fitness and Growth Potential

The tolerance model rests on the thesis of a physiologically regulated, albeit unsustainable, systemic attempt to adapt to the catabolic challenge posed by acute prepubescent malnutrition even in its severe forms. The model centers on the immunological component of the attempt, positing reorientation toward a non-inflammatory form of competence in place of the classic paradigm of immunological attrition and exhaustion. The foundation of the model was laid in 1990, and sixteen years later it was articulated formally on the basis of a body of evidence centered on T cell cytokines and interventions with cytokine and hormonal mediators. The benefit originally suggested was a reduced risk of autoimmune pathologies consequent to the catabolic release of self-antigens, hence the designation highlighting immune tolerance. Herein, the emergence of the tolerance model is traced from its roots in the recognition that acute malnutrition elicits an endocrine-based systemic adaptive attempt. Thereafter, the growth of the evidence base supporting the model is outlined, and its potential to shed new light on existing information is tested by application to the findings of a published clinical study of acutely malnourished children. Finally, some knowledge gaps pertinent to the model are identified and its potential for growth consonant with evolving perceptions of immunobiology is illustrated.

Fidelity in Animal Modeling: Prerequisite for a Mechanistic Research Front Relevant to the Inflammatory Incompetence of Acute Pediatric Malnutrition

Inflammatory incompetence is characteristic of acute pediatric protein-energy malnutrition, but its underlying mechanisms remain obscure. Perhaps substantially because the research front lacks the driving force of a scholarly unifying hypothesis, it is adrift and research activity is declining. A body of animal-based research points to a unifying paradigm, the Tolerance Model, with some potential to offer coherence and a mechanistic impetus to the field. However, reasonable skepticism prevails regarding the relevance of animal models of acute pediatric malnutrition; consequently, the fundamental contributions of the animal-based component of this research front are largely overlooked. Design-related modifications to improve the relevance of animal modeling in this research front include, most notably, prioritizing essential features of pediatric malnutrition pathology rather than dietary minutiae specific to infants and children, selecting windows of experimental animal development that correspond to targeted stages of pediatric immunological ontogeny, and controlling for ontogeny-related confounders. In addition, important opportunities are presented by newer tools including the immunologically humanized mouse and outbred stocks exhibiting a magnitude of genetic heterogeneity comparable to that of human populations. Sound animal modeling is within our grasp to stimulate and support a mechanistic research front relevant to the immunological problems that accompany acute pediatric malnutrition.

Elevated blood interleukin-10 levels and undiminished systemic interleukin-10 production rate prevail throughout acute protein-energy malnutrition in the weanling mouse

The objectives were to determine if blood IL-10 levels rise during the early stages of acute (wasting) pre-pubescent malnutrition in metabolically distinct murine models known to depress inflammatory immune competence and whether systemic IL-10 production is affected in these pathologies. Weanling C57BL/6J mice were assigned to dietary protocols that elicited wasting pathologies mimicking the human diseases of marasmus (restricted-intake group) or incipient kwashiorkor (low-protein group). An age-matched control group also was included. Serum IL-10 bioactivities were assessed in the early (day 3) and advanced (day 14) stages of weight loss, and net systemic IL-10 production was assessed at the same stages of pathology by in vivo cytokine capture. Blood IL-10 levels were elevated in both malnourished groups relative to controls at days 3 and 14 (range of P values: 0.03-0.0001). Further, despite a limited supply of energy and nitrogenous substrates, the systemic IL-10 production rate was at least sustained in the malnourished groups and, in fact, was elevated in the marasmic group (P=0.05) throughout the progression of weight loss. IL-10 emerges as an anti-inflammatory mediator positioned to participate in initiating and upholding the depressed immune competence that accompanies acute pre-pubescent deficits of protein and energy.

Effect of the inhibition of triiodothyronine (T3) production by thiamazole on the T3 and serotonin content of immune cells

Triiodothyronine (T3) and serotonin are present in the cells of immune system (blood, peritoneal and thymic lymphocytes, monocytes and granulocytes of the blood and peritoneal fluid, mast cells). In the present experiments the effect of thiamazole, an antithyroid drug was studied on the content of these two hormones in immune cells after one and two weeks of continuous treatment (by drinking water, containing 30 mg/100 ml thiamazole, ad libitum) in adult male rats, using flow cytometric and confocal microscopic analysis. In thymic lymphocytes both hormone contents significantly increased in both time points. A significant decrease of T3 was observed in peritoneal monocytes and granulocytes also in both time points, in peritoneal mast cells after one week and in blood lymphocytes after two weeks. Serotonin content in addition to the elevated thymic values (in both measurements) was significantly reduced in blood lymphocytes after two weeks. Confocal microscopy demonstrated heterogeneous cell populations with disparate hormone content and mostly diffuse localization The experiments call attention to the presence of T3 in the immune cells and to its variable concentration under the effect of a thyrostatic drug as well, as to the T3-serotonin relationship in the cells of the immune system.

The capacity of noninflammatory (steady-state) dendritic cells to present antigen in the primary response is preserved in acutely protein- or energy-deficient weanling mice

The objective of this investigation was to determine the influence of wasting protein and/or energy deficits on the capacity of dendritic cells to initiate primary responses. Weanling male and female C57BL/6J mice were permitted free access to a complete purified diet, free access to an isocaloric low protein purified diet (combined deficiencies of protein and energy) or restricted intake of the complete diet (energy deficiency) for up to 14 d; a 19-d-old zero-time control group was also included. Malnourished mice lost 1.5-2% of initial body weight daily. Antigen presentation by dendritic cells from spleen and lymph nodes was assessed in vitro by the primary one-way allogeneic mixed lymphocyte reaction using CBA/J mononuclear or CD4(+) T cells as responders. This function was sustained despite advanced weight loss and, remarkably, was increased in cell suspensions from 14-d energy-deficient mice. Antigen presentation by dendritic cells in mononuclear suspensions was examined in vivo using the host-vs.-graft response in CBA/J recipients, and an ontogeny-related increase was sustained in both malnourished groups through 14 d of weight loss. Neither wasting protocol influenced the proportion of mononuclear cells (1-2%) exhibiting dendritic cell phenotype (CD11c(+)F4/80(-/low)) in the cellular suspensions used to study antigen-presenting activity. Consequently, these functional studies are interpretable on a per dendritic cell basis. In the absence of infectious or inflammatory pressure, the dendritic cell retains antigen-presenting capacity despite acute (wasting) deficiencies of protein and/or energy. The results are relevant to presentation of both foreign (adjuvant role) and self (tolerizing role) antigens by the dendritic cell.

The proportion of CD45RA(+)CD62L(+) (quiescent-phenotype) T cells within the CD8(+) subset increases in advanced weight loss in the protein- or energy-deficient weanling mouse

Male and female C57BL/6J mice, initially 19 d old, had free access to a complete purified diet, were fed this diet in restricted daily quantities, or had free access to a low-protein diet. Three separate studies were conducted with feeding periods of 14, 9 or 6 d (n = 7-8 per dietary group and feeding period; 6 d: restricted intake and age-matched controls only). A zero-time control group (19 d old) was included in each study. Malnourished mice lost approximately 2% of initial body weight daily. Naïve-phenotype (quiescent) CD8(+) T cells of the blood, spleen and mesenteric lymph nodes were identified on the basis of surface coexpression of CD45RA and CD62L. Relative to age-matched controls, the percentage of naïve-phenotype CD8(+) T cells was high in energy-restricted groups after 9 d and 14 d of weight loss and in the protein-restricted groups after 14 d (P < or = 0.05). No ontogenetic change was apparent (age-matched vs. zero-time control). Other studies have demonstrated depression in cell-mediated immune competence in both malnutrition models within the first week of weight loss. An overabundance of quiescent-phenotype T cells within the involuted CD8(+) compartment may contribute to established immune depression but not to its initiation in weight loss pathologies.

Overabundance of CD45RA(+) (quiescent-phenotype) cells within the involuted CD4(+) T-cell population follows initiation of immune depression in energy-deficient weanling mice and reflects involution exclusive to the CD45RA(-) subset

Previous studies have identified an overabundance of quiescent-phenotype (CD45RA(+)) CD4(+) T cells throughout the lymphoid system of weanling mice at an advanced stage of food intake restriction mimicking marasmus. The objective of this investigation was to determine the timing of this phenomenon relative to the development of depression in cell-mediated immune competence. Two experiments were conducted in which male and female weanling C57BL/6J mice, initially 19 d of age, either were permitted free access to a complete purified diet or were subjected to restricted intake of this diet, producing loss of 1.5-2% of initial body weight daily. In the first experiment, feeding periods of 3, 6, 9, 12 and 14 d were examined, and a zero-time control group (19 d old) was also included. Expression of CD45RA was assessed by flow cytometry in CD4(+) T cells from the blood, spleen and mesenteric lymph nodes. Despite reduction in CD4(+) T-cell numbers, evident in all three lymphoid compartments of the malnourished mice by d 6, energy-restricted mice maintained the numbers of CD4(+)CD45RA(+) T cells at the level found in the zero-time control group. Consequently, the malnourished group exhibited a high percentage of CD4(+) T cells expressing CD45RA by d 9 in the blood and mesenteric nodes and by d 12 in the spleen. In the second study, malnourished and age-matched control groups were sensitized to sheep red blood cells on d 3 and energy-restricted mice exhibited depression in the delayed hypersensitivity response to this antigen when assessed on d 9 after challenge 24 h previously. Energy deficiency pathology includes a shift toward CD4(+) T cell quiescence that may contribute to ongoing immunodepression without being involved in its initiation. Remarkably, this imbalance develops because involution of the CD4(+) subset in the energy-deficient mice is confined to the CD45RA(-) population.

T cells with a quiescent phenotype (CD45RA+) are overabundant in the blood and involuted lymphoid tissues in wasting protein and energy deficiencies

The objective of this investigation was to determine the influence of distinct forms of acute weight loss on the expression of the quiescence marker, CD45RA, by T cells in several lymphoid compartments including the blood. Male and female weanling mice, CBA/J and C57BL/6J strains, were allocated to the following groups: ad libitum intake of a complete purified diet; restricted intake of the complete diet; and ad libitum intake of an isocaloric low-protein diet. The restricted intake protocol produced weight loss through energy deficiency (marasmic-type malnutrition), whereas the low-protein diet caused wasting through inadequate protein nitrogen and induced a condition mimicking incipient kwashiorkor. In one experiment, weanling mice of both strains were maintained for 14 days according to each of these dietary protocols and, in a supplementary study, C57BL/6J weanlings consumed either the complete diet or the low-protein diet ad libitum for 21 days. Zero-time control groups (19-days old and 23-days old in C57BL/6J and CBA/J strains, respectively) were included in the first experiment to control for ontogeny-related change. Expression of CD45RA was assessed by two-colour flow cytometry in CD4+ and CD8+ T cells from the spleen, mesenteric lymph nodes and blood. Within 14 days, energy-restricted mice exhibited a high percentage of CD4+ T cells expressing CD45RA in all three lymphoid compartments in both mouse strains (an average of 50% CD45RA+ versus 9% in well-nourished controls), and a similar outcome was apparent in the CD8+ subset (93% CD45RA+ versus 63%). Mice fed the low-protein diet required up to 21 days to exhibit the same imbalance within the CD4+ T-cell subset (33% CD45RA+ versus 4% in well-nourished controls). A shift toward a quiescent phenotype occurs throughout the peripheral T-cell system in acute wasting disease. Consequently, the quiescence-activation phenotype of blood T cells reflects the same index in secondary lymphoid organs in such pathologies. Naive-type quiescence among T cells is implicated as a component of depressed adaptive immunocompetence in the advanced stages of diverse forms of acute weight loss.

Effects of triiodothyronine supplements on splenic natural killer cells in malnourished weanling mice

The main purpose of this investigation was to determine whether exogenous triiodothyronine (T3) administered according to a protocol known to prevent depression in acquired immunity in weanling murine protein-energy malnutrition (PEM) would, likewise, influence the splenic natural killer (NK) cell in this disease. Weanling mice of disparate inbred strains, C57BL/6J and CBA/J, were subjected to wasting PEM produced by means of two low-protein diets (0.5% crude protein) identical in every respect except that one diet contained supplemental T3 (0.2 micrograms/g diet). NK cell lytic activity toward YAC-1 targets was assessed in vitro using suspensions containing 0.5 x 10(6) mononuclear spleen cells. Lytic activity in this assay was low in mice fed the unsupplemented low-protein diet, but was not depressed in malnourished animals given exogenous T3. Surface marker analysis using the NK cell-specific antigen, NK 1.1 (PK 136), revealed no effect of the low-protein diet or of exogenous T3 on the proportion of splenic mononuclear cells exhibiting NK 1.1+ phenotype. Previous investigations have shown that acquired immune competence in PEM can be manipulated, by means of endocrine hormonal intervention, independently of continued wasting disease. The present results extend this fundamental new concept to include an innate immune function, namely NK cell lytic activity. In this system of experimental PEM, exogenous T3 prevented depression in NK cell lytic activity expressed on a per cell basis. The malnourished weanling rodent is a particularly powerful experimental system with which to investigate the mechanisms whereby thyroid hormones influence NK cells.