Pulmonary delivery of interleukin-7 provides efficient and safe delivery to the aging immune system

Abstract

Age-associated atrophy of the thymus with coincident reduction in thymopoeisis, decline in thymic output, and subsequent immune dysfunction has been reversed by the use of interleukin-7 (IL-7). In the earlier studies and in clinical trials, delivery of IL-7 has been by multiple injections over several days to maintain effective activity levels in the tissues. This is unlikely to meet with high compliance rates in future clinical use, and so we tested alternate routes of delivery using a technique involving tagging IL-7 with fluorescent dye that emits in the near-infrared region and whose fluorescence can be visualized within the tissues of live animals. We have shown that intratracheal instillation, enabling transfer through the lungs, provides an effective route for delivering IL-7 into the bloodstream and from there into the tissues in older animals. Delivery is rapid and widespread tissue distribution is seen. Comparison of administration either subcutaneously or by instillation reveals that IL-7 delivery by the pulmonary route provides significantly greater transmission to lymphoid tissues when compared with injection. In functional assessment studies, pulmonary administration led to significantly improved intrathymic T cell development in older animals when compared with IL-7 delivered by injection. Furthermore, in these older animals, delivery of IL-7 by intratracheal instillation was not accompanied by any apparent adverse events when compared with controls receiving saline vehicle by instillation or animals receiving IL-7 by subcutaneous injection.

FIG. 1.

Fluorescent imaging of distribution of labeled interleukin-7 (IL-7) in a young animals within the lung approximately 60 min after intratracheal instillation of 500 ng of labeled IL-7.

FIG. 2.

Detection of fluorescence in single young animal at timed intervals after the instillation or injection of labeled interleukin-7 (IL-7). Each image is a ventral view and the approximate time after administration is shown at the side of each image.

FIG. 3.

Relative fluorescent signal detected from peripheral blood samples in young animals taken over a 48-hr time period using different routes of administration of labeled interleukin-7 (IL-7) or the free carboxylate dye control. Figures are the average values obtained from between two to five samples. Free carboxylate dye was instilled into the lungs (open triangles) or injected subcutaneously (open circles) and compared with tagged IL-7 instilled into the lungs (closed circles) or injected subcutaneously (closed triangles).

FIG. 4.

Relative fluorescent signal per milligram of tissue detected from different organs of young animals receiving labeled interleukin-7 (IL-7) (shaded columns) or carboxylate dye (open columns) by instillation or injection.

FIG. 5.

Representative examples of the fluorescent profiles obtained for the subsets defined by expression of CD44 and CD25 within the CD3⁻CD4⁻CD8⁻ population from older animals who received interleukin-7 (IL-7) (A) or phosphate-buffered saline (PBS) (B) through the instillation route or IL-7 (C) or PBS (D) by injection.

FIG. 6.

Pie chart showing the distribution of the minor subsets within the CD3⁻CD4⁻CD8⁻ population, showing the averages and the percentage contribution of n=7 (interleukin-7 [IL-7] instilled shown in A) and n=8 (phosphate-buffered saline [PBS] instilled shown in B) older animals. Total numbers of the CD3⁻CD4⁻CD8⁻population are shown below each pie chart.

FIG. 7.

Comparison between interleukin-7 (IL-7)–instilled and IL-7–injected older animals of the numbers of cells within each minor subset of the CD3⁻CD4⁻CD8⁻ population.

FIG. 8.

Relative weight change in older animals receiving interleukin-7 (IL-7) by instillation (n=7) at times during the procedure (A) and comparison of the total number of mononuclear cells within the large left lung lobe of mice (B) treated by instillation or injection. PBS, Phosphate-buffered saline.