Mary Jo Turk

Characterization of a novel pH-sensitive peptide that enhances drug release from folate-targeted liposomes at endosomal pHs

Although liposomes have proven useful for the delivery of drugs and gene therapy vectors, their potencies are often compromised by poor unloading following uptake into their target cells. We have consequently explored the properties of a novel 29-residue amphipathic peptide that was designed by arrangement of hydrophobic and hydrophilic residues to disrupt liposomes at lower peptide concentrations than previously tested peptides. The peptide was indeed found to promote pH-dependent liposome unloading with improved efficiency. A peptide of the same sequence, but half the length, however, promoted pH-dependent permeabilization only at much higher concentrations. Further characterization of the longer peptide revealed that release of liposome contents (i) occurred at a pH of approximately 6, (ii) became less efficient as the size of the encapsulated cargo increased, and (iii) was moderately suppressed in cholesterol-containing liposomes. Use of this peptide to enhance the cytotoxicity of cytosine arabinoside encapsulated in folate-targeted liposomes demonstrated an increase in drug potency of approximately 30-fold. Gene expression by a serum-stable folate-targeted liposomal vector was also measurably enhanced by inclusion of the peptide. We conclude that intracellular unloading of liposomal contents can be significantly improved by co-encapsulation of an optimally designed, pH-sensitive peptide.

Multiple pathways to tumor immunity and concomitant autoimmunity

Summary: The immune repertoire contains T cells and B cells that can recognize autologous cancer cells. This repertoire is directed against self, and in some cases altered self (mutations). Priming immune responses against self antigens can be difficult. Strategies are presented using altered self to elicit immunity against self in poorly immunogenic tumor models. Mechanisms underlying immunity to self antigens on cancer cells show that the immune system can use diverse strategies for cancer immunity, in both the immunization and the effector phases. CD4+ T cells are typically, but not always, required for immunization. The effector phase of tumor immunity can involve cytotoxic T cells, macrophages with activating Fc receptors, and/or killer domain molecules. This diversity in the effector phase is observed even when immunizing with conserved paralogs. A consequence of tumor immunity is potentially autoimmunity, which may be undesirable. Autoimmunity uses similar mechanisms as tumor immunity, but tumor immunity and autoimmunity can uncouple. These studies open up strategies for active immunization against cancer.

The Effects of pH and Intraliposomal Buffer Strength on the Rate of Liposome Content Release and Intracellular Drug Delivery

Targeted liposomal drug formulations may enter cells by receptor-mediated endocytosis and then traffick by membrane flow into acidic intracellular compartments. In order to understand the impact of these intracellular pH changes on liposomal drug unloading, the effect of pH on the release from folate-targeted liposomes of three model compounds with distinct pH dependencies was examined. 5(6)-carboxyfluorescein, which titrates from its anionic to uncharged form following internalization by KB cells, displays strong endocytosis-dependent release, since only its uncharged (endosomal) form is membrane permeable. Endocytosis-triggered unloading of drugs of this sort is enhanced by encapsulating the drug in a weak buffer at neutral pH, so that acidification of the intraliposomal compartment following cellular uptake can occur rapidly. Sulforhodamine B, in contrast, retains both anionic and cationic charges at endosomal pH (~pH 5), and consequently, escapes the endosomes only very slowly. Doxorubicin, which is commonly loaded into liposomes in its membrane-impermeable (cationic) form using an acidic buffer, still displays endocytosis-triggered unloading, since sufficient uncharged doxorubicin remains at endosomal pHs to allow rapid re-equilibration of the drug according to the new proton gradient across the membrane. In this case, when the extraliposomal [H+] increases 250-fold from 4 × 10−8 M (pH 7.4, outside the cell) to 10−5 M (pH 5, inside the endosome), the ratio of doxorubicin inside to outside the liposome must decrease by a factor of 250. Therefore, the collapse of the transliposomal pH gradient indirectly drives an efflux of the drug molecule from the liposome. Since a change in intraliposomal pH is not required to unload drugs of this type, the intraliposomal compartment can be buffered strongly at acidic pH to prevent premature release of the drug outside the cell. In summary, pH triggered release of liposome-encapsulated drugs can be achieved both with drugs that increase as well as decrease their membrane permeabilities upon acidification, as long as the intraliposomal buffer strength and pH is rationally selected.

Immunity to Cancer Through Immune Recognition of Altered Self: Studies with Melanoma

The adaptive immune system is capable of recognizing cancer through T- and B-cell receptors. However, priming adaptive immunity against self antigens is potentially a difficult task. Presentation of altered self to the immune system is a strategy to elicit immunity against poorly immunogenic antigens. We have shown that immunization with conserved paralogues of tumor antigens can induce adaptive immunity against self antigens expressed by cancer. Remarkably, cancer immunity elicited by closely related paralogues can generate distinct adaptive immune responses, either antibody or T-cell dependent. Cancer immunity induced by xenogeneic immunization follows multiple and alternative pathways. The effector phase of tumor immunity can be mediated by cytotoxic T cells or macrophages and perhaps natural killer cells for antibody-dependent immunity. Helper CD4+ T cells are typically, but not always, required to generate immunity. Autoimmunity is frequently observed following immunization. Cancer immunity and autoimmunity use overlapping mechanisms, and therefore they are difficult to uncouple, but distinct pathways can be discerned that open the eventual possibility of uncoupling tumor immunity from autoimmunity. Studies examining the molecular basis for immunogenicity of conserved paralogues are facilitating the development of new strategies to rationally design vaccines that trigger adaptive immune responses to cancer.

GM-CSF DNA induces specific patterns of cytokines and chemokines in the skin: implications for DNA vaccines

Granulocyte-macrophage colony-stimulating factor (GM-CSF) enhances immune responses by inducing the proliferation, maturation, and migration of dendritic cells, and the expansion and differentiation of B and T lymphocytes. Similar biological effects have been observed with the use of GM-CSF DNA in mouse models for therapy of cancer and infectious diseases, and its use is currently being investigated in clinical trials in combination with DNA vaccines. To further understand the adjuvant mechanisms of GM-CSF DNA, we examined early events following its administration. We found measurable levels of GM-CSF protein in the skin and muscle, as well as in serum. Measurements of other cytokine and chemokine levels revealed differential expression patterns over time. The early response was characterized by high levels of inflammatory molecules, including IL-1beta, IL-6, TNFalpha, RANTES, MIP-1alpha and MCP-1, later followed by expression of precursor Th1 cytokines, IL-12 and IL-18, concomitant with IFNgamma production. Local production of GM-CSF protein also resulted in the early recruitment of polymorphonuclear cells and later recruitment of mononuclear cells, including dendritic cells. These results have implications for understanding early events in the immune response to DNA vaccines, and provide a basis for development of new approaches to cancer vaccines, including the use of cytokine genes as adjuvants.