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Natural killer (NK) cells can spontaneously lyse certain virally infected and transformed cells. However, early in immune responses NK cells are further activated and recruited to tissue sites where they perform effector functions. This process is dependent on cytokines, but it is unclear if it is regulated by NK cell recognition of susceptible target cells. We show here that infiltration of activated NK cells into the peritoneal cavity in response to tumor cells is controlled by the tumor major histocompatibility complex (MHC) class I phenotype. Tumor cells lacking appropriate MHC class I expression induced NK cell infiltration, cytotoxic activation, and induction of transcription of interferon γ in NK cells. The induction of these responses was inhibited by restoration of tumor cell MHC class I expression. The NK cells responding to MHC class I–deficient tumor cells were ∼10 times as active as endogenous NK cells on a per cell basis. Although these effector cells showed a typical NK specificity in that they preferentially killed MHC class I–deficient cells, this specificity was even more distinct during induction of the intraperitoneal response. Observations are discussed in relation to a possible adaptive component of the NK response, i.e., recruitment/activation in response to challenges that only NK cells are able to neutralize.

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The role of CD4+ and CD8+ T cells in the surveillance of Trypanosoma cruzi or Trypanosoma brucei brucei was studied in mice which lacked CD4 or CD8 molecules and which were generated by embryonic stem cell technology. Whereas wild-type mice infected with T. cruzi (Tulahuen strain) displayed low levels of parasitemia and no mortality, striking increases in parasite growth and mortality occurred in both CD8and CD4mice. On the contrary, CD8and, to a lesser degree, CD4mice showed enhanced resistance to T. b. brucei. T-cell-dependent immunoglobulin G-specific responses were produced in CD8but not CD4mice. Normal T-cell proliferative responses were measured in both CD4and CD8mice. Interleukin4 production after concanavalin A or anti-CD3 monoclonal antibody stimulation was strikingly enhanced in CD8-but not CD4spleen cells, whereas gamma interferon production was normal in both CD4and CD8spleen cells. Spleen and lymph node cells from CD8-(but not CD4-) mice at 20 days postinfection with T. cruzi had higher levels of interleukin4 mRNA than the wild-type controls, as shown in a competitive polymerase chain reaction assay. On the other hand, CD4-(but not CD8-) mice at 20 days postinfection with T. cruzi had lower levels of gamma interferon mRNA than the wild-type mice.

Generation of natural killer (NK) cells in spleens from radiation chimeras produced between pairs of histocompatible 'high' and 'low' NK-reactive mouse strains has been investigated. Spleen cells of high-reactive recipients reconstituted with bone marrow from low-reactive mice were found to be low reactive. Conversely, spleen cells of low mice grafted with bone marrow or fetal liver cells from high donors were high reactive. Similarly, the age-related changes of NK activity were shown to be expressed at the bone marrow precursor cell level. These results indicate that the generation of natural killer cells is an inborn and autonomous function of the bone marrow and does not depend on the genotype or other influences of the host environment.

The present work describes the purification and characterization of antigen B (AgB), the thermostable lipoprotein from E. granulosus. Native AgB was purified to homogeneity by a new strategy involving adsorption on DEAE-Sepharose, followed by immunopurification. The purified antigen was analysed using mapped monoclonal antibodies (MoAbs) and peptide isolation by in situ digestion in gels after SDS-PAGE. Epitope mapping of 7 MoAbs using PEPSCAN, synthetic peptides and competition studies, revealed that six of them defined epitopes which clustered the N-terminal extension of a 8 kDa subunit of AgB, whilst the remaining one reacted against the stretch RGLIAEGE, corresponding to the C-terminus. The epitopes defined by the seven MoAbs were found to be present in all the subunits. Furthermore, the similarities of the peptide finger prints obtained by HPLC analysis and amino acid sequencing of tryptic peptides isolated from the 8, 16 and 24 kDa subunits, indicated that they have most if not all the amino acid sequence in common. We also found evidence that the band representing a component of an apparent molecular weight of 8 kDa in SDS-PAGE, believed to be the smallest subunit of AgB, contained at least two components, which may constitute the building blocks of the higher molecular weight subunits.

T lymphocytes provide a major line of defence against many protozoan parasites. The aim of this work was to determine the role of T-cell helper/inducer subset (T h/i) in the resistance to Trypanosoma cruzi in a murine model. The importance of natural killer (NK) cells in the resistance to the parasite was also evaluated. BALB/c and C57BL/6 mice were injected with either monoclonal antibodies against L3T4, Thy 1.2, NK1.1, or with a polyclonal rabbit antiserum against NK cells (anti-asialo GM-1). The effect of in vivo administration of these antibodies was tested in separate functional assays. After antibody treatment, mice were infected with a low dose of T. cruzi in the bloodstream form. Mice depleted of, or reduced in T, T h/i, or NK cell activity all developed higher parasitaemia and had higher mortality than their control counterparts. Mice injected with anti-L3T4 monoclonal antibodies were unable to generate a specific antibody response to the parasite. Treatment of mice with alpha/beta interferon, which is known to boost NK cell activity, resulted in an enhanced resistance to the parasite. Our data indicate that T h/i cells as well as NK cells are of vital importance in controlling parasitaemia and reducing mortality in T. cruzi-infected mice. Finally, we also demonstrate that the production of antibodies specific for T. cruzi is strictly T helper cell-dependent.