Acquisition of autoimmunity genes by New Zealand mice is associated with natural resistance to infection by mycobacteria

NOD mice are resistant to depletion of thymic cells caused by acute stress or infection

NOD mice spontaneously develop autoimmune diabetes; disease onset in females of our colony of NOD mice usually takes place around the 4th month of age. Diabetes of NOD mice can be modulated by different stress protocols, even though these animals were shown to be resistant to the effects of glucocorticoids on their lymphocytes. We have recently found that the early host inflammatory response to mycobacteria can be strongly modified by stress, the autoimmunity-prone NOD and NZB/W mice being particularly affected. These mice show reduced numbers of granulocytes in the inflammatory cavity after exposure to stress. Mycobacterium avium is an opportunistic agent that is responsible for disseminated infections seen in AIDS patients. Here, we investigated whether the early immune response to M. avium was altered by stress in NOD mice and we compared the stress response of these mice with a non-autoimmune strain, BALB/c mice. The effects of stress on infected BALB/c mice, which like AIDS patients are susceptible to M. avium infection, offers experimental evidence that M. avium infection, if coupled with stress of the host, may accelerate loss of T helper cells. In contrast, in NOD mice, stress or infection significantly increased the number of cells of the thymuses of the animals. Data obtained with NOD mice support the previously reported resistance of NOD mice lymphocytes to glucocorticoids and suggest that there are two distinct signalling pathways involved in the response of NOD lymphocytes to these stress hormones: one leading to apoptosis and the other mediating glucocorticoid inhibition of activation-induced cell death.

Diabetes-prone NOD mice are resistant to Mycobacterium avium and the infection prevents autoimmune disease

It was recently proposed that the diabetes genes of non-obese diabetic (NOD) mice are linked to the Bcg gene that is associated with resistance to infection by mycobacteria; however, it has not been established whether NOD mice are resistant or susceptible to the infection, although there are previous investigations on response of NOD mice to other intracellular parasites (e.g. Kaye et al., Eur. J. Immunol. 22: 357-364). We have investigated here this question, as well as the consequences of mycobacterial infection on the natural history of murine diabetes. Female NOD mice were intraperitoneally infected with 10(8) viable bacilli of Mycobacterium avium at 2 months of age, i.e. before the mice show diabetes; they were studied up to the sixth month of age (when more than half of untreated female NOD mice show glycosuria). To determine whether NOD mice were susceptible or resistant to M. avium infection, we have compared the kinetics of bacterial growths in liver and spleen of the mice with those determined in M. avium-susceptible (BALB/c) and resistant (C3H) strains of mice. NOD mice were able to control the M. avium infection, following a pattern similar to that observed in infected C3H mice. The mycobacterial infection prevented the expression of diabetes in all of the infected NOD mice and it also decreased the incidence of proteinuria in the treated mice. The infected NOD mice showed a marked enhancement in antibodies against the 65,000 mycobacteria antigen (heat-shock protein (hsp) 65) up to the second month of infection and these elevated titres slowly decreased in the following months; anti-hsp 65 antibodies were not detected in age-matched controls. This is the first demonstration that NOD mice are naturally resistant to mycobacterial infection, and we reinforce evidence on the role of immune response triggered by mycobacteria and its hsp 65 antigen in prevention of diabetes in NOD mice.

Stress modulates acute inflammation triggered by mycobacteria in autoimmunity-prone and normal mice

We have investigated the effect of a single stress treatment of mice on the early cellular response (granulocyte influx) of acute inflammation that was produced by intraperitoneal inoculation of the animals with mycobacteria (10(6) bacilli of M. avium). Autoimmunity-prone (NZB/W) and normal (BALB/c, MRL, NZW) mice were submitted to the same stress (20 min of swimming) this given either before or simultaneously with the induction of the intraperitoneal mycobacteria-induced inflammation. Local inflammation was evaluated by the quantification of leukocytes harvested from the peritoneal cavity of the mice; this was done 30 and 60 minutes after the end of the stress treatment. We found that the stressor alone was able to increase the number of cells that were harvested from the peritoneal cavity; this may be due to stress-induced detachment of resident macrophages from the peritoneal walls. The autoimmunity-prone mice (NZB/W) showed a marked decrease in the number of inflammatory cells (mostly of granulocytes) when stress treatment was performed immediately before the triggering of inflammation; these stress-induced alterations were attenuated in normal mice (BALB/c and MRL strains), as well as in the non-autoimmune parent strain (NSW) of NZB/W mice. Modulation of the acute inflammatory response by stress was smaller if the stress was induced concomitantly with the triggering of inflammation; here, NZB/W were again the mice most affected by stress. Our data indicate that [1] stress is able to modify the acute inflammatory response of mice; [2] autoimmunity-prone mice are more sensitive to stress-induced modulation of inflammation than normal animals; and [3] the timing of stress (with regard to the initiation of inflammatory phenomena) is an important factor in the intensity of changes produced by stress in the early cellular response of acute inflammation.

Mycobacteria-induced autoantibody production is associated with susceptibility to infection but not with host propensity to develop autoimmune disease

Mycobacteria cause increase in autoantibody production in the host during the first weeks of infection. The level of the autoantibody enhancement varies widely in different hosts, suggesting that it depends on features of the host make-up. We have investigated the participation of two characteristics of the host in the modulation of mycobacteria-induced autoantibody production: (i) the host being genetically determined to later develop spontaneous autoimmune disease; (ii) the host being susceptible/resistant to mycobacterial infection. Mycobacterium avium infection was studied in 3-month-old mice that are prone (NZB and C57B1/6-lpr/lpr strains) or not (NZW and C.D2 strains) to develop, when older, autoimmune disease; these murine strains are either naturally susceptible (C57B1/6-lpr/lpr and NZW) or resistant (NZB and C.D2) to mycobacteria. Mycobacterium avium infection was produced by i.p. injection of 3 x 10(7) viable bacilli. At days 15 and 30 of the infection, we determined the following parameters: (i) number of cells producing natural autoantibodies (splenic cells showing surface antibodies against bromelain-treated mouse (BrM) erythrocytes); (ii) suppression of the primary response to T cell-dependent antigen (i.e. to sheep erythrocytes); (iii) immunoglobulin classes and IgG isotypes; (iv) titres of anti-dsDNA antibodies; and (v) serum concentrations of interferon-gamma (IFN-gamma). We found that the highest elevations in natural autoantibodies were associated with hosts being naturally susceptible to mycobacteria, but not with the host being genetically determined to later develop autoimmune disease. The rise in autoantibodies was predominantly of the IgM type, being associated with suppression of the T cell response and accompanied by increase in serum IFN-gamma. Mycobacteria failed to induce any significant enhancement in pathogenic anti-dsDNA antibodies. Our data suggest that the finding of a high level of autoantibodies during the early phase of mycobacterial infection reflects host susceptibility of the infectious agent, and that it is not related with its propensity to later develop autoimmune disorders.

Effect of mycobacterial infection in the lupus-prone MRL/lpr mice: enhancement of life span of autoimmune mice, amelioration of kidney disease and transient decrease in host resistance

We have used the MRL/lpr murine model of spontaneous lupus to investigate three questions in infection immunity: (1) does mycobacterial infection have any effect on the mortality of the autoimmunity-prone lpr mice?; (2) does the infection modify the progression of kidney disease of lpr lupus?; and (3) does the lpr gene change the resistance of mice to mycobacteria? Experimental infections were induced by intraperitoneal inoculation of 10(7) viable bacilli of Mycobacterium avium in 3 months old MRL/lpr mice and also in congeneic MRL/+ mice (lacking the lpr gene). MRL mice were sacrificed 1, 2.5 and 4 months after the M. avium injection. We found that infection caused lowering of urine protein concentration in lpr mice as compared with age-matched lpr controls. Mycobacteriosis also induced a marked decrease in the mortality of lpr animals, e.g. 85% of infected lpr mice reached the age of 7 months whereas only 10% of control lpr mice reached the same age. MRL/lpr mice showed, after 1 and 2.5 months of infection, higher mycobacterial loads than the congeneic non-lpr MRL mice; after 4 months of infection, however, differences in M. avium loads between the two groups of MRL mice became not statistically significant. We conclude that: (1) mycobacterial infection increases the life span of lpr mice; (2) the infection slows down the progression of kidney disease in the lupus-prone lpr animals; (3) the autoimmunity gene lpr is associated with a transient decrease in host resistance to mycobacteria.