Mechanisms of immunity in typhus infection: analysis of immunity to Rickettsia mooseri infection of guinea pigs

Murine Typhus: A Review of a Reemerging Flea-Borne Rickettsiosis with Potential for Neurologic Manifestations and Sequalae

Murine typhus is an acute febrile illness caused by Rickettsia typhi, an obligately intracellular Gram-negative coccobacillus. Rats (Rattus species) and their fleas (Xenopsylla cheopis) serve as the reservoir and vector of R. typhi, respectively. Humans become infected when R. typhi-infected flea feces are rubbed into flea bite wounds or onto mucous membranes. The disease is endemic throughout much of the world, especially in tropical and subtropical seaboard regions where rats are common. Murine typhus is reemerging as an important cause of febrile illness in Texas and Southern California, where an alternate transmission cycle likely involves opossums (Didelphis virginiana) and cat fleas (Ctenocephalides felis). Although primarily an undifferentiated febrile illness, a range of neurologic manifestations may occur, especially when treatment is delayed. Serology is the mainstay of diagnostic testing, but confirmation usually requires demonstrating seroconversion or a fourfold increase in antibody titer from acute- and convalescent-phase sera (antibodies are seldom detectable in the first week of illness). Thus, early empiric treatment with doxycycline, the drug of choice, is imperative. The purpose of this review is to highlight murine typhus as an important emerging and reemerging infectious disease, review its neurologic manifestations, and discuss areas in need of further study.

Rickettsial vaccines: the old and the new

In the past century vaccine development for prevention of rickettsial diseases has been prolific. However, in the past 20 years no new rickettsial vaccine has been manufactured and there are currently no new or old rickettsial vaccines licensed. Early rickettsial vaccines were difficult, expensive and very hazardous to produce. Molecular biology techniques of today are currently being used to develop new rickettsial vaccines that are standardized, inexpensive, nonhazardous and efficacious.

Depletion of gamma interferon and tumor necrosis factor alpha in mice with Rickettsia conorii-infected endothelium: impairment of rickettsicidal nitric oxide production resulting in fatal, overwhelming rickettsial disease

C3H/HeN mice infected intravenously with a dose of Rickettsia conorii (Malish 7 strain) that is sublethal for immunocompetent animals (1.1 x 10(3) PFU) developed disseminated infection of endothelial cells of the brain, lungs, heart, liver, kidney, testis, and testicular adnexa. In R. conorii-infected mice depleted of gamma interferon (IFN-gamma) and/or tumor necrosis factor alpha (TNF-alpha) by intravenous administration of neutralizing monoclonal antibodies on days 0, 2, and 4, the mortality rate was 100%. Death of the cytokine-depleted animals on days 5 and 6 was associated with overwhelming rickettsial infection documented by titration of rickettsial content in the brain and liver and by immunohistologic demonstration of massive quantities of R. conorii in endothelial cells of all organs examined, in macrophages of the liver and spleen, and in hepatocytes. Nondepleted, immunocompetent animals showed markedly reduced rickettsial content in the tissues on day 6, with rickettsial destruction in phagolysosomes not only in macrophages but also in endothelial cells and hepatocytes. All nondepleted, infected mice recovered and appeared completely healthy by day 9. Assay of liver infiltrated by lymphocytes and macrophages revealed mRNA of IFN-gamma and TNF-alpha, indicating that the host defenses were activated at the site of infection. Treatment of mice with an analog of L-arginine reduced the synthesis of nitric oxide and impaired rickettsial killing. Nitric oxide production was also impaired in cytokine-depleted infected mice. These observations support the hypothesis that IFN-gamma secreted by T lymphocytes and natural killer cells and TNF-alpha secreted by macrophages act in a synergistic, paracrine fashion on adjacent rickettsia-infected endothelial cells, hepatocytes, and macrophages to stimulate synthesis of nitric oxide, which kills intracellular R. conorii.

Demonstration and partial characterization of antigens of Rickettsia rhipicephali that induce cross-reactive cellular and humoral immune responses to Rickettsia rickettsii

The relatively unrelated spotted fever group rickettsia Rickettsia rhipicephali conferred on guinea pigs protective immunity against challenge with virulent R. rickettsii. Immunity was conferred at all doses of R. rhipicephali used in the study. Because of the serologic unrelatedness of these two rickettsiae, determined by the use of microimmunofluorescence and other serological assays, further studies were performed to define the nature of the immune response elicited by R. rhipicephali and the characteristics of the rickettsial antigens that evoke cross-reactive antibody responses. Animals immune to R. rhipicephali tested at the time of challenge showed a complete cross-reactive lymphocyte proliferative response to rickettsial antigens prepared from each species. In fact, spleen cells from R. rhipicephali-immune animals responded better to R. rickettsii antigens than to homologous immunizing antigens. Serum samples were obtained from R. rhipicephali-infected animals at various times after infection and tested by the use of Western immunoblot assay for antibodies that were cross-reactive with antigens of R. rickettsii. By 10 days after infection with R. rhipicephali, antibodies to antigens of both species were noted, and by 37 days after infection, sera from immune animals showed strong reactivity to antigens of R. rhipicephali with apparent molecular masses of 107 and 151 kDa. The cross-reactive antibody response to antigens of R. rickettsii was relatively strong and involved predominantly the rOmpB protein and the rickettsial lipopolysaccharide. These findings establish the presence of T-cell-dependent epitopes associated with antigens of R. rhipicephali, which confer protective immunity against challenge with R. rickettsii. Results of Western immunoblot assays support the contention that the R. rickettsii rOmpB surface antigen contains important protective epitopes.

The importance of the crystalline surface layer protein antigens of rickettsiae in T-cell immunity

Studies in animal models have demonstrated that solid immunity to typhus rickettsiae is dependent on immune T cells. In addition, the surface protein antigen (SPA) of typhus rickettsiae has been shown to be an effective immunogen, protecting vaccinated animals against subsequent challenge with virulent organisms. In the present studies we describe three classes of human lymphocytes which are capable of lysing cells infected with typhus rickettsiae. The first class is CD3,8-positive and is capable of specially lysing both HLA-matched and mismatched targets infected with typhus rickettsiae. Since this cytotoxic effector can be generated with IL-2 as well as with SPA it appears to be a lymphokine-activated killer (LAK). The second class of lymphocytes is CD3,4-positive and is capable of producing gamma interferon in response to the SPA of typhus rickettsiae. Gamma interferon in turn can cause the lysis of cells infected with typhus rickettsiae as well as inhibit intracellular rickettsial growth. A third cytotoxic effector which is CD3,4-positive and which is capable of lysing only HLA-matched targets infected with typhus rickettsiae was generated with a sonicated antigen, and its fine antigenic specificity is not known at present. We conclude that the SPA is an immunologically important protein for the human host and represents an outstanding candidate for a subunit vaccine against typhus.

Gamma interferon as a crucial host defense against Rickettsia conorii in vivo

Gamma interferon (IFN-gamma) plays an important role as a host defense in rickettsial infection. Swiss Webster mice, which are resistant to Rickettsia conorii (Malish 7 strain) infection, were treated with a monoclonal antibody against mouse IFN-gamma. When the antibody-treated mice were inoculated with 12 50% tissue culture infective doses of R. conorii, the mortality was 47% and the morbidity was 100%. None of the control mice, which received the same dose of R. conorii, died or became ill. The enumeration of rickettsiae in organs by direct immunofluorescence in paraffin sections demonstrated higher quantities of rickettsiae in the spleen had liver of IFN-gamma-depleted mice as compared with those of the infected controls. The kinetic analysis of IFN-gamma levels in sera showed depletion in the treated mice. These results indicate that IFN-gamma plays an important role as a host defense in the early stage of rickettsial infection. Survival of some mice despite continued treatment with antibody to IFN-gamma suggests that other immune mechanisms may also be important.

Characteristics of lymphoid cells that adoptively transfer immunity to Rickettsia mooseri infection in mice

The capacity of adoptively transferred immune lymphoid cells or passively transferred immune serum to alter the course of an established Rickettsia mooseri (R. typhi) infection in the spleen was evaluated in BALB/c mice. Immune cells, but not immune serum, controlled the established infection. An effective lymphocyte was a T-cell which had to possess a capacity to divide.

Characteristics of Rickettsia mooseri infection of normal and immune mice

Rickettsia mooseri infection initiated by subcutaneous injection has been studied in BALB/c mice with the objective of developing a model for the study of immune mechanisms. Characterization of infection included the following: measurement of the replication, dissemination, and clearance of rickettsiae; measurement of correlates of the immune response, including humoral antibody, hypersensitivity to subcutaneously inoculated rickettsial antigen, and activation of nonspecific macrophage microbicidal capacity; and measurement of resistance to a second homologous challenge. Local infection at the site of subcutaneous injection progressed through day 5 and was controlled by day 7. Systemic infection as determined by the presence of rickettsiae in spleen was first detected on day 7 and progressed through day 14; however, rickettsiae persisted in this organ at reduced numbers through at least day 28. Control of the local infection at the site of subcutaneous injection occurred at about the time humoral antibodies and hypersensitivity reactions to subcutaneously injected rickettsial antigens became demonstrable and was paralleled by a capacity to resist homologous subcutaneous challenge at a site distant from that of the primary infection. Systemic infection progressed in spite of this acquired immune capacity and was controlled in the spleen in parallel with the development of enhanced macrophage microbicidal capacity in the liver. The results show that an acquired immunity is capable of restricting rickettsial growth at subcutaneous sites at a time when rickettsiae are increasing in titer in deep organs.

Cloned mouse interferon-gamma inhibits the growth of Rickettsia prowazekii in cultured mouse fibroblasts

The effect of treating cultured mouse fibroblasts (L929 cells) with cloned mouse interferon-gamma on the growth of Rickettsia prowazekii within the fibroblasts was studied. Within 48 h after infection, rickettsiae were cleared from a substantial proportion of the initially infected cells and rickettsial growth was inhibited in those cells that remained infected, when L929 cells were treated with cloned mouse interferon-gamma both before and after infection. When L929 cells were treated with cloned mouse interferon-gamma either only before or only after infection with rickettsiae, rickettsial growth was markedly inhibited but rickettsiae were not cleared from many cells. Addition of cycloheximide to L929 cells markedly suppressed the antirickettsial activity of the interferon, and cloned mouse interferon-gamma did not induce antirickettsial activity in human foreskin fibroblasts. The antirickettsial effects of cloned mouse interferon-gamma were similar to those induced by crude mouse lymphokines prepared from concanavalin A-stimulated mouse spleen cells. Equivalent amounts (units) of cloned mouse interferon-gamma produced by Chinese hamster ovary cells or by Escherichia coli caused equivalent inhibition of rickettsial growth in mouse fibroblasts. However, at high concentrations of interferon-gamma, treatment of rickettsia-infected fibroblasts with equivalent amounts (units) of interferon-gamma, as crude mouse lymphokines or cloned mouse interferon-gamma, resulted in slightly greater inhibition of rickettsial growth by the crude lymphokines. Most of the antirickettsial activity of crude mouse lymphokines can be explained by the interferon-gamma that is present in these preparations. Interferon-gamma, by virtue of its ability to inhibit rickettsial growth and effect the clearance of rickettsia from nonprofessional phagocytes, may play a crucial role in the elimination of rickettsiae from the infected host.

Inhibition of the growth of Rickettsia prowazekii in cultured fibroblasts by lymphokines

The effect of lymphokine treatment of mouse and human fibroblast cell lines on the growth of Rickettsia prowazekii within the fibroblasts was studied. Treatment of mouse L929 cells with concanavalin A- or antigen-induced mouse lymphokines both before and after infection with R. prowazekii led to clearance of the rickettsiae from a substantial proportion of the cells and suppression of rickettsial growth in those cells which remained infected. Similar but less dramatic anti-rickettsial effects were observed in L929 cells treated with mouse lymphokines either only before or after infection with rickettsiae. Mouse lymphokine treatment of L929 cells had similar anti-rickettsial effects on the avirulent E strain and the virulent Breinl strain of R. prowazekii. Addition of cycloheximide or emetine to L929 cells at the same time as the lymphokines markedly suppressed the inhibition of rickettsial growth by the lymphokines. Mouse lymphokine treatment inhibited rickettsial survival and growth in mouse 3T3-A31 cells as well as in mouse L929 cells, but had no effect on rickettsial survival and growth in human foreskin fibroblasts. Conversely, concanavalin A-induced human lymphokines inhibited rickettsial survival and growth in human foreskin fibroblasts but had no effect on rickettsial survival and growth in mouse L929 cells. The rickettsia inhibitory activity in concanavalin A-induced mouse lymphokines was destroyed by heating the lymphokines at 80 degrees C for 10 min or by holding the lymphokines at pH 2 for 24 h but was retained after heating at 56 degrees C for 30 min.