Immune and nonimmune regulation of the population of Trypanosoma musculi in infected host mice

Rodent trypanosomes: their conflict with the immune system of the host

Rodent trypanosome infections provide the opportunity to study major phenomena that are displayed in many parasite-host combinations and, therefore, the chance to contribute to the elucidation of those phenomena. Here, Julia and Joseph Albright focus on the immune responses to rodent trypanosomes and on the tricks the parasites play to minimize the effects of those responses.

T cell-dependent elimination of dividing Trypanosoma grosi from the bloodstream of Mongolian jirds

Mongolian jirds, Meriones unguiculatus, are susceptible to infection with Trypanosoma grosi, which naturally parasitizes Apodemus spp. The present study investigated T cell dependence of elimination of T. grosi from the bloodstream of jirds by in vivo T cell depletion using a monoclonal antibody (HUSM-M.g.15). In T cell-depleted jirds, elimination of T. grosi, particularly the dividing forms, from the bloodstream was significantly delayed, occurring at around week 3 p.i. The kinetics of serum levels of IgM and IgG specific to trypanosomes in T cell-depleted and control immunocompetent jirds were different; peak levels of IgM were noted on days 6-8 p.i. around the time of peak parasitaemia (day 6 p.i.) in immunocompetent jirds, whereas the serum levels began to increase abruptly after day 10 p.i., peaking at around day 18 p.i. in T cell-depleted jirds. Similarly, serum IgG increased after day 6 p.i. in immunocompetent jirds, in contrast to after day 12 p.i. in T cell-depleted jirds, and the level increased steadily even after disappearance of parasitaemia. Our findings indicate that T cells play a major role at least in the 'first crisis' during elimination of dividing T. grosi from the bloodstream.

Susceptibility of natural hybrids between house mouse subspecies to Sarcocystis muris

A previous study showed that the susceptibility of hybrids between two Mus musculus musculus and Mus musculus domesticus did not apply to every parasite. The authors proposed that only parasites which exerted enough constraints would induce the selection of poorly compatible systems of resistance in the subspecies. This study completes the previous work. Experimental infections of mice of the two subspecies and their hybrids with the tissue-dwelling protozoan Sarcocystis muris show that hybrids are more susceptible to the coccidian than M. m. musculus and M. m. domesticus individuals. This result demonstrates that the hybrids are not only susceptible to intestinal helminths, and confirms the 'constraint hypothesis'.

Experimental trypanosomiasis of natural hybrids between house mouse subspecies

This study characterises the extent of the susceptibility to parasites (first demonstrated with helminths) of hybrids between Mus musculus domesticus and Mus musculus musculus. Experimental infections with Trypanosoma musculi of M. m. domesticus, M. m. musculus and their natural hybrids have been performed to compare their level of resistance/susceptibility. It appears that contrary to the results with helminths, hybrid mice present the same level of resistance/susceptibility to the trypanosome as M. m. musculus and M. m. domesticus individuals. This result is interpreted in the light of the modalities of host parasite interactions and leads us to hypothesise on the role of parasitism in the evolution of the house mouse hybrid zone.

Trypanosoma musculi: tracking parasites and circulating lymphoid cells in host mice

Two aspects of host-parasite relationships that seem worthy of more attention are: (a) the distribution of parasites among host organs in the early course of infection, and (b) the dynamics of host lymphocyte tissue localization and recirculation during the course of infection. We have employed the derivatized aminostyrylpyridinium dye, [125I] I 2P-Di-6-ASP, to provide a relatively stable tag on both a parasite, Trypanosoma musculi, and on host mouse splenocytes, enriched B and T lymphocytes, and natural killer cells. The organ distribution of the parasites, splenocytes, and lymphocytes in recipient, host mice was tracked. Radiolabeled T. musculi localized primarily in the liver with lesser numbers in spleen, lungs, and kidneys. Per unit wet weight, the spleen accumulated parasites most efficiently. When T. musculi were inoculated intraperitoneally, most of them remained in the peritoneal space and the numbers that gained access to liver, lungs, and spleen were significantly smaller than in mice inoculated intravenously. The acquisition of parasites by the spleen (and lungs) of mice with an existing T. musculi infection was markedly inhibited. This was true also of syngeneic splenocytes and lymphocytes. In addition, lymphocytes from infected mice were significantly less likely to take residence in the spleens of normal recipient mice and were especially unlikely to localize in the spleens of infected recipients. These and other findings suggested that the inability of circulating lymphocytes to gain access to lymphoid tissues in infected mice, coupled with the poor ability of those tissues to sequester parasite antigens, could account for the known prolonged delay in the development of curative antibody response characteristic of T. musculi-infected mice. It is likely that the marked disruption of lymphoid tissue histoarchitecture that is typical of T. musculi infection contributes significantly to the failure of the tissues to sequester parasites and lymphocytes. Because lymphoid tissue disruption is seen in many parasitic infections, the findings reported here may have fairly broad relevance. In any case, the procedure described here for labeling parasites and lymphocytes should be of general utility for tracking their disposition in vivo.

Impaired natural killer cell function as a consequence of aging

Alex Comfort wrote (1979): "... nobody needs long-lived mice." His point was, of course, that as much as possible, research should be done with humans "... who are the beneficiaries in mind ..." In this paper we hope to show that long-lived mice have been useful, if not essential, for conducting studies on the aging of innate immunity, specifically the NK cell component of the system. NK cells are activated early in the course of Trypanosoma musculi infections, which we employ as a model. We have generated evidence that the relatively severe infections of aged mice with T. musculi, are attributable, in part, to (a) functionally defective NK cells, the defect(s) being retained by LAK cells that arise from them, and (b) deficient amounts of IL-2 required to convert NK to LAK cells. Defective macrophages, which are the effector cells responsible for eliminating T. musculi, may also accumulate in aged animals. We postulate that the functionally deficient NK cells fail to generate adequate amounts of IFN gamma (and perhaps, TNF alpha) to optimally activate macrophages. This inadequacy can explain the weak ability of aged mice to control the early stage of T. musculi infection preceding the appearance of the more slowly-developing acquired immune response.

Innate control of the early course of infection in mice inoculated with Trypanosoma musculi

Infections of mice with Trypanosoma musculi result in marked suppression of acquired humoral immunity but rapid activation of splenic NK cell cytotoxicity. We show that both NK cells and activated peritoneal space (PS) macrophages (MP) participate in the innate immune control of T. musculi infections preceding escape of curative antibody production from suppression. Splenic NK cytotoxicity reaches a peak on Days 3-4 of infection and then rapidly declines. Rising cytotoxicity is paralleled by a rising number of NK cells. The decline in cytotoxicity occurs even though the number of splenic NK cells continues to rise. The critical role of NK cells in the control of the early course of T. musculi infection was demonstrated by the effects of either depleting NK cells (antiasialo GM1 treatment) or maintaining them in an activated state (poly(I:C) injections). The importance of MP in controlling the infection was suggested by studies involving proteose peptone elicited MP both in vivo and in culture. The results presented here strongly suggest that innate immunity involving NK cells and MP can control, but not cure, T. musculi infections. Whether this early innate response influences the subsequent acquired, curative response remains to be studied. Detailed analyses of innate immunity in this experimental infection should suggest new approaches to intervention in early pathogenic infections.

Response of inbred mice to infection with a new isolate of Trypanosoma musculi

A new isolate of Trypanosoma musculi has been derived from organisms recovered from Mus spretus captured in Lisbon, Portugal. The time-course and profiles of infection with this isolate in inbred mice have been compared with those obtained with the existing Partinico II isolate. Infections with the Portuguese isolate are less intense, and controlled more quickly than those with the Partinico isolate. As with the latter, there are marked mouse strain-dependent influences on infection with the Portuguese isolate, but these strain-dependent characteristics differ considerably with each isolate. For example, NIH mice were the most susceptible to the Partinico II isolate, but virtually refractory to the Portuguese isolate. Mice exposed to infection with one isolate show complete immunity to both homologous and heterologous challenge infections. These striking interactions between host and parasite genotype are discussed in terms of immunological influences on infection.