Cross-presentation of cell-bound Ags from established, solid tumors to CD8 cells is efficient and likely to have a role in determining host response to tumor. A number of investigators have predicted that when tumor Ags are derived from apoptotic cells either no response, due to Ag “sequestration,” or CD8 cross-tolerance would ensue. Because the crucial issue of whether this happens in vivo has never been addressed, we induced apoptosis of established hemagglutinin (HA)-transfected AB1 tumors in BALB/c mice using the apoptosis-inducing reagent gemcitabine. This shrank the tumor by ∼80%. This induction of apoptosis increased cross-presentation of HA to CD8 cells yet neither gross deletion nor functional tolerance of HA-specific CD8 cells were observed, based on tetramer analysis, proliferation of specific CD8 T cells, and in vivo CTL activity. Interestingly, apoptosis primed the host for a strong antitumor response to a second, virus-generated HA-specific signal in that administration of an HA-expressing virus after gemcitabine administration markedly decreased tumor growth compared with viral administration without gemcitabine. Thus tumor cell apoptosis in vivo neither sequesters tumor Ags nor cross-tolerizes tumor-specific CD8 cells. This observation has fundamental consequences for the development of tumor immunotherapy protocols and for understanding T cell reactivity to tumors and the in vivo immune responses to apoptotic cells.

A number of tumor studies have indicated a link between CD4 help and the magnitude and persistence of CTL activity; however, the mechanisms underlying this have been largely unclear. To evaluate and determine the mechanisms by which CD4+ T cells synergize with CD8+ T cells to prevent tumor growth, we used the novel technique of monitoring in vivo CTL by labeling target cells with CFSE. This approach was supported by the direct visualization of CTL using peptide-MHC tetramers to follow tumor-specific T cells. The data presented demonstrate that while cotransfer of Ag-specific CD4+ T cells was not required for the generation of CTLs, because adoptive transfer of CD8+ T cells alone was sufficient, CD4+ T cells were required for the maintenance of CD8+ T cell numbers. Our data suggest that there is a correlation among the number of CD8+ T cells, in vivo CTL function, and IFN-γ production, with no evidence of a partial or nonresponsive phenotype among tetramer-positive cells. We also show that CD4+ T cells are required for CD8+ T cell infiltration of the tumor.

The plant cytotoxin ricin consists of two disulfide-linked chains, each of about 30,000 daltons. An initial model based on a 2.8 A MIR electron density map has been refined against 2.5 A data using rounds of hand rebuilding coupled with either a restrained least squares algorithm or molecular dynamics (XPLOR). The last model (9) has an R factor of 21.6% and RMS deviations from standard bond lengths and angles of 0.021 A and 4.67 degrees, respectively. Refinement required several peptide segments in the original model to be adjusted translationally along the electron density. A wide range of lesser changes were also made. The RMS deviation of backbone atoms between the original and model 9 was 1.89 A. Molecular dynamics proved to be a very powerful refinement tool. However, tests showed that it could not replace human intervention in making adjustments such as local translations of the peptide chain. The R factor is not a completely satisfactory indicator of refinement progress; difference Fouriers, when observed carefully, may be a better monitor.

The protein CheZ, which has the last unknown structure in the Escherichia coli chemotaxis pathway, stimulates the dephosphorylation of the response regulator CheY by an unknown mechanism. Here we report the co-crystal structure of CheZ with CheY, Mg(2+) and the phosphoryl analog, BeF(3)(-). The predominant structural feature of the CheZ dimer is a long four-helix bundle composed of two helices from each monomer. The side chain of Gln 147 of CheZ inserts into the CheY active site and is essential to the dephosphorylation activity of CheZ. Gln 147 may orient a water molecule for nucleophilic attack, similar to the role of the conserved Gln residue in the RAS family of GTPases. Similarities between the CheY[bond] CheZ and Spo0F [bond]Spo0B structures suggest a general mode of interaction for modulation of response regulator phosphorylation chemistry.

Ricin has been refined in a crystallographic sense to 2.5 A resolution and the model for the A-chain (RTA) is described in detail. Because RTA is the first member of the class of plant toxins to be analyzed, this model probably defines the major structural characteristics of the entire family of these medically important proteins. Explanations are provided to rationalize amino acids that are conserved between RTA and a number of homologous plant and bacterial toxins. Eight invariant residues appear to be involved in creating or stabilizing the active site. In the active site Arg180 and Glu177 are hydrogen bonded to each other and also coordinate a water molecule; each of these groups may be important in the N-glycosidation reaction. Several other polar residues may play lesser roles in the mechanism, including tyrosines 80 and 123 and asparagines 78 and 209. A number of conserved hydrophobic residues are seen to cluster within several patches and probably drive the overall folding of the toxin molecule.