Immunotherapy for Infectious Diseases

Dendritic Cells 107
CD8� CTLs, this occurs inefficiently. Since CD8� T-cell mediated immunity is
required to control most infections, the use of DC pulsed with full-length proteins is
not currently favored.
Several approaches have been developed to solve this problem, including gene
transfer, which has been shown to result in antigen processing in the HLA class I pathway
of DCs and presentation to CD8� T-cells. Recombinant viral and bacterial vectors
such as adenovirus, vaccinia, fowlpox, salmonella, and listeria expressing the
antigen have been used successfully to deliver transgene products into the HLA class
I pathway of DCs (121–125). Another approach involves the use of “transporter” peptides
to bring proteins into APCs for processing. Conjugating such peptides onto fulllength
proteins or peptides allows these antigens to translocate across cell membranes
and into the HLA class I pathway. One such peptide is obtained from HIV tat. Tat is
an 86-amino acid protein that has been shown to be rapidly transported from extracellular
milieu into the cytosol of most cells. This property presumably plays an important
role in viral replication or spread. However, it might also provide the means of
delivering antigens into APCs for processing and presentation. When full-length tat
protein was conjugated to galactosidase, horseradish peroxidase, RNAase, or
pseudomonas exotoxin, the conjugated proteins crossed the cellular membrane of
fibroblasts with enzymatic activity intact. One problem that must be overcome if tat is
to be used for antigen delivery is that full-length tat protein as well as large (�20
amino acids) peptide fragments of tat are highly cytotoxic. We have addressed this
issue by identifying the minimal region of tat required for transfer into cytosol and
have demonstrated that a short, basic sequence corresponding to residues 49–57 enters
cells without affecting viability (126) and induces both CD8� and CD4� T-cell
responses. HIV-1 tat can increase the efficiency of HLA class I-restricted antigen presentation
by more than 100-fold (126).
CLINICAL TRIALS OF DENDRITIC CELL IMMUNOTHERAPY
HIV Trials
Numerous studies in animal models have documented that ex vivo antigen-pulsed
DCs are effective inducers of pathogen-specific immunity (59,63,66,67). However, the
utility of ex vivo antigen-pulsed DCs for the prophylaxis or therapy of infection has
not yet been extensively studied in humans. In the first reported DC clinical trial in
HIV-infected patients, the safety and antigen-presenting properties of allogeneic or
autologous DCs were investigated in seven HLA-A2�, HIV-infected patients (46).
Allogeneic DCs, obtained from the peripheral blood of HLA-identical, HIV-seronegative
siblings using the density gradient procedure described earlier, were pulsed with
recombinant HIV-1 MN gp160 or synthetic peptides corresponding to HLA-A*0201restricted
cytotoxic epitopes of envelope, Gag and Pol proteins. The antigen-pulsed
cells were infused intravenously six to nine times at monthly intervals, and HIV-specific
immune responses were monitored.
One allogeneic DC recipient with a CD4� T-cell count of 460/mm 3 showed increases
in envelope-specific CTLs and lymphocyte proliferative responses, as well as IFN-� and
IL-2 production. Two other allogeneic DC recipients with CD4� T-cell counts of 434 and
560/mm 3 , respectively, also showed an increase in HIV envelope-specific lymphocyte