The immune privilege of the subretinal space has been a
subject of recent interest
1 2 because of the potential of
allogeneic retinal transplantation in the treatment of several diseases
for which we currently have inadequate therapies, such as hereditary
retinal degeneration (e.g., retinitis pigmentosa) and age-related
macular degeneration. The well-defined immunogenetics of the rodent
make it an attractive species in which to conduct such experiments,
particularly because some rodent models of hereditary retinal
degeneration resemble human disease and have been well characterized
(e.g., the
rd mouse and Royal College of Surgeons rat).
However, the small size of the rodent eye and the large size of the
rodent lens severely compromise the ability of that species to provide
a definitive answer to this problem.
First, the large lens and small vitreous cavity (<5 μl) of the
rodent eye require a transscleral (or more anatomically disruptive)
approach to deposition of tissue or antigen in the subretinal space. A
transvitreal approach is not anatomically possible. Disruption of the
integrity of the retinal pigment epithelium (RPE) must occur with a
transscleral approach to the subretinal space. Thus, an important
anatomic barrier for the establishment of immunologic privilege in the
subretinal space will be breached, with the possible compromise of the
immunologic properties of this site. Second, the deposition of tissue
or antigen in the subretinal space using a transscleral approach does
not allow the investigator to control carefully the volume of inoculum
placed in the space—much of it invariably leaks out of the scleral
entrance site—or to place the inoculum precisely in the subretinal
space and not in surrounding compartments.
In a review of the light micrographs in the article by Zhang and
Bok,
1 allogeneic RPE cells appear to be in the subretinal
space as well as in the choroid. The presence of allogeneic cells in
the latter would severely compromise a study of the immunologic
privilege of the subretinal space.
We have used several different surgical modifications of the
transscleral approach in the rodent, with many different instruments
and needles, and have not been satisfied that we can control either the
quantity or the placement of the inoculum satisfactorily.
Figure 1 depicts a rat eye after the injection of 3 μl india ink into the
subretinal space. The ink is delivered through a 30-gauge blunt cannula
attached to a Hamilton syringe, through a small radial sclerotomy
anterior to the equator made with a myringotomy blade. Although part of
the inoculum is in the subretinal space
(Fig 1A) , it can also be seen
in the choroid and vitreous cavity on adjacent histologic sections
(
Figs. 1B 1C , respectively).
We believe it is critical for any study of immune privilege in the
subretinal space to demonstrate convincingly the ability to place a
precise quantity of the inoculum in that space, without contamination
of the adjacent compartments. Even with such demonstrated expertise,
the conclusions reached must be qualified by the anatomic breach of the
RPE barrier. It is for these reasons, as well as others, that we
believe an analysis of immune privilege in the subretinal space that
can be meaningfully translated to allogeneic retinal transplantation in
man must be performed using a transvitreal approach in a larger
species.