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Summary. Many solid tumours have been shown to lack expression of either of the immune co-stimulatory molecules CD80 (B7AE1) or CD86 (B7AE2), which is thought to be one of the ways in which tumours may escape immune recognition. We have examined the surface expression of CD80, CD86, human leucocyte antigen (HLA) class I and II, CD11a, CD54, and CD58 on the blast cells from patients with acute myeloid leukaemia (AML) at presentation. CD80 was only rarely expressed on AML blasts and, in those leukaemic cells expressing CD80, the level of expression was low. In contrast, expression of CD86 was detected on the AML blasts in more than half of the samples tested and, in some cases, the level of expression was equivalent to that of mature monocytes and activated B lymphocytes. The percentage of leukaemic blasts expressing CD86 was higher in the M4 and M5 French-American-British (FAB) types, and expression of CD11a and HLA class II was higher in the M4 FAB type. There was no difference in expression of CD80, CD54, CD58, or HLA Class I between different FAB subgroups. There was no significant difference in duration of first remission with expression of CD80, CD86, CD11a, CD54 or HLA class II. However, when expression of CD80 and CD86 were considered together, a significantly longer duration of remission was found. We suggest that these molecules may play a role in immunosurveillance, resulting in prolonged remission in some patients treated for AML.

Interleukin-12 (IL-12) is a cytokine that exhibits pleiotropic effects on lymphocytes and natural killer cells and has been shown to have promise for the immunotherapy of cancer. The combination of the immune costimulatory molecule B7.1 and IL-12 has been shown to be synergistic for T cell activation. By transfecting tumor cells with both IL-12 and B7.1 cDNAs, it may be possible to use these modified targets as vaccines. A major obstacle in designing a vector to deliver these genes results from the structure of IL-12. Functional IL-12 is a heterodimer composed of two distinct subunits that are encoded by separate genes on different chromosomes. Production of functional IL-12 requires the coordinated expression of both genes. This presents several problems in vectors, particularly those in which additional genes, either a co-stimulatory gene or a selectable marker, are inserted. Therefore, we have constructed a single cDNA that encodes a single-chain protein, called Flexi-12, which retains all of the biological characteristics of recombinant IL-12 (rIL-12). The monomeric polypeptide Flexi-12 is able to induce the proliferation of phytohemagglutinin (PHA) blasts, induce PHA blasts to secrete interferon-gamma (IFN-gamma) and additionally, by preincubation, enhance the killing of K562 targets by PBLs. These phenomena are in a dose-dependent manner comparable to that seen with rIL-12. We have also shown that tyrosine phosphorylation of the STAT 4 transcription factor, which has been shown to be unique to the IL-12 signaling pathway, occurs with Flexi-12 at levels similar to those seen with rIL-12. We have packaged Flexi-12 into a recombinant adeno-associated virus (AAV) and used this vector to infect acute myeloid leukemic (AML) blasts. Infected AML blasts produced between 2 and 6 ng of IL-12/10(6) cells per ml per 48 hr. These studies also confirm that AAV is an efficient delivery vehicle for cytokines to leukemic cells. Direct analysis of these modified cells acting as tumor vaccines is underway.