Immunological responses are tightly controlled through a balance between stimulatory and inhibitory cytokines. This is used therapeutically in chronic inflammatory diseases such as rheumatoid arthritis or Crohn’s disease, where anti-cytokine therapy has proven to be useful. Furthermore, stimulatory cytokines are used in treatment of some malignant diseases. However, both anti-cytokine therapy and cytokine therapy are systemic treatments with severe side-effects: thus local delivery of the treatment may improve the therapeutic efficacy and reduce the systemic side-effects.

We therefore propose the concept of local delivery using secretory lysosomes of hematopoietic cells as vehicles. A secretory lysosome is a compartment with a dual function of storage and degradation found in secretory cells, e.g. in neutrophils and lymphocytes. The secretory lysosomes are released by degranulation at sites of inflammation or infection. Therapeutically-active protein could be induced for expression and storage in secretory lysosomes and thereby be locally released by degranulation resulting in local delivery of therapeutically active protein.

In support of this concept, we have shown that: I) gene transfer and granule loading can be achieved using the soluble TNF-a receptor (sTNFR1) after cDNA expression in hematopoietic cell lines and hematopoietic progenitor cells; II) endoplasmic reticulum export can be facilitated by the addition of a transmembrane domain; III) constitutive secretion can be prevented by incorporating a cytosol-sorting signal resulting in secretory lysosome targeting; IV) the sTNFR1 is released from the transmembrane domain into the secretory lysosome by proteolytic cleavage; V) regulated sTNFR1-secretion can be triggered by activation or calcium signal.

In vivo investigations are currently determining the feasibility of local protein delivery at sites of inflammation or malignancy.

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