In vitro stimulation of human peripheral blood lymphocytes by soluble and membrane fractions of renografin-purified typhus group rickettsiae

Activation and enhanced contact of human T-lymphocytes with autologous red blood cells are required for their stable adherence at 37 degrees

The adherence of human red blood cells (RBC) to autologous T-cells does not occur in the body, and in vitro is elicited at 4 degrees. Autologous E-rosetting at 37 degrees has not previously been described. In this work, lymphocyte-RBC adherence has been studied in mixed leukocyte-RBC cultures and in whole blood from healthy donors. Vital, cytochemical and electron microscopic studies have shown that T-cells may form stable E-rosettes with autologous RBC at 37 degrees. As in the previously reported cold-dependent reversible rosetting, stable rosetting is mediated by the erythrocyte LFA3 and lymphocyte CD2 molecules. Uniquely, this phenomenon requires both T-cell activation and an enhanced contact between the T-cell and RBC membranes. These requirements were met by exposure of cell cultures to: (1) PHAE, the erythroagglutinating component of PHAP, or (2) to either non-erythroagglutinating mitogens, PHAL, Con A, OKT3 or SEA, or to antigens of typhus group rickettsiae or salmonellae, provided that the RBC membrane was desialyted. Cultures derived from individuals seropositive to rickettsiae or vaccinated with salmonellae demonstrated the adherence phenomenon after antigen exposure when neuraminidase was present in the culture medium. The system 2 described here can be used as a diagnostic tool for defining activated T-cells and T-cell clones with the memory to antigens capable of inducing cell-mediated immunity.

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.

Heterogeneity of CD4-positive human T-cell clones which recognize the surface protein antigen of Rickettsia typhi

Immunity to the typhus group of rickettsiae is largely dependent on the effector function of several classes of T lymphocytes, including those which produce gamma interferon. Since the surface protein antigen (SPA) derived from typhus group rickettsiae has been shown to be an effective immunogen in animal models, human T-cell clones specific for the SPA of Rickettsia typhi were isolated and tested for their antigenic specificity, as well as for their ability to produce gamma interferon. Eighteen CD4-positive clones specific for the SPA of R. typhi exhibited considerable diversity in their response to the SPAs derived from two strains of Rickettsia prowazekii and from Rickettsia canada. The vast majority of clones also recognized the SPAs from R. prowazekii but not from R. canada. Two heteroclitic clones demonstrated significantly higher proliferative responses to the SPAs derived from one or both of the R. prowazekii strains than to the SPA of R. typhi, and one clone demonstrated a significantly higher response to the SPA of R. typhi than to the other SPAs. All 18 clones produced gamma interferon in response to SPA stimulation. We conclude that the SPAs from typhus group rickettsiae can elicit both a diverse T-cell response in humans and the efficient stimulation of gamma interferon-mediated immunity.

Use of a sensitive microplate enzyme-linked immunosorbent assay in a retrospective serological analysis of a laboratory population at risk to infection with typhus group rickettsiae

A microplate enzyme-linked immunosorbent assay (ELISA), developed for the detection of antibodies to typhus group rickettsiae, was used to analyze human sera from individuals engaged directly or indirectly in rickettsial research. The earliest serum available from each of 112 individuals was tested for immunoglobulin M (IgM) and IgG antibodies against Rickettsia typhi and Rickettsia prowazekii by ELISA at a 1:500 dilution. In at least one assay, nine sera had ELISA optical densities of greater than 0.2, which were above the mean optical densities plus three standard deviations of the other 103 sera. Three of the positive sera were from individuals with known clinical cases of typhus infection. The other sera with predominantly IgG titers were from individuals with extended laboratory exposure to rickettsiae or histories of typhus vaccination, or both. During continued serological surveillance, eight additional people with repeated occupational exposure to typhus rickettsiae had seroconversions in the ELISA to optical densities of greater than 0.2. No apparent clinical illness occurred in two individuals, whereas six clinical cases of infection occurred in others subsequent to accidental laboratory autoinoculation (one) or aerosol exposures (five). In the clinical infections, antibodies were first detected at 7 days, but in subsequent sera, rises and declines in titers were quite variable and were influenced by vaccination, relapse, and time and extent of antibiotic therapy. In primary infections the sera of several individuals who received immediate antibiotic therapy had brief strong IgM responses without pronounced increases in IgG. In contrast, much higher IgG levels were attained in three cases in which relapse occurred, the individual had previously been immunized, or treatment had been delayed. The microplate ELISA proved to be a highly sensitive and reliable test for detection of the human serological response to typhus antigens.