Recombinant DNA Advisoiy Committee - 2/10-11/92 
needed. To overcome this obstacle, a PCR can be used to rapidly identify whether the 
infused cells exist as long as there is a marker amenable to the use of the PCR primers. 
In HIV, there are potential toxicities associated with the infusion of T-cell clones as with 
many other viral diseases. Therefore, it is not possible to generalize that all T-cell 
responses to all viruses are beneficial. 
In HIV, there are a couple of settings in which an efficient T-cell response to the virus is 
potentially problematic. Infusing T-cell clones which are very reactive to the viral antigens 
can result in lysis of all recognizable target cells. There is a potential interval for 
exuberant response, minus some toxicity. HIV is capable of infecting hematopoietic stem 
cells, which are what repopulates the patient bone marrow. If the patient produces a very 
efficient response capable of ablating all those cells, the bone marrow is eliminated and 
death ensues. Also, the HIV virus can infect microglial cells in the brain. There are a 
number of murine models in which the T-cell response to a control nervous system (CNS) 
infection causes CNS dysfunction. There are data that suggest that this response could 
occur with HIV infection of microglia in the brain when there is an over exuberant T-cell 
response. If the patients experience exuberant response and significant CNS dysfunction 
after the T-cell infusion, there should be a way to ablate these T-cells. 
Dr. Greenberg described lymphocytic pneumonia, where the over exuberant immune 
response causes too much inflammation. The HIV patients experience inflammation of 
the lung which can cause obliteration of the air sac compromising the ability of the lung to 
exchange oxygen. If this reaction occurs, T-cells could be ablated after administration. On 
this premise, a retrovirus was constructed in which a single protein is driven off the viral 
LTR conferring both positive and negative selection. Also, the T-cells expressing 
hygromycin resistance would be expressing the HSV-TK function; therefore, the selection 
for antibiotic resistance selects for the expression of the "suicide" gene. 
Dr. Greenberg described an example of T-cell clones, specific for HIV, that were 
transduced with HyTK and exhibited sensitivity to ganciclovir. Both the non-transfected 
parent clones and the transfected HyTK clones survived in the absence of ganciclovir. 
When exposed to relatively low doses of ganciclovir, virtually all HyTK clones were dead 
in six days. The doses of ganciclovir used in vitro are below those levels that are 
achievable in vivo. 
Dr. Greenberg addressed selection of patients who would receive ganciclovir and how it 
would affect their normal therapy. At one point, all patients who were undergoing BMT 
were placed on a protocol in which they received ganciclovir prophylaxis to prevent CMV 
infection. Results demonstrated that ganciclovir had substantial toxicity where there was 
bone marrow suppression. Patients demonstrating bone marrow suppression would not be 
eligible to receive ganciclovir. The only patients eligible to receive ganciclovir are those 
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Recombinant DNA Research, Volume 15 
