Crop plants are being used to produce a range of vaccines and drugs including powerful molecules that affect cells of the immune system. The inevitable contamination of our food crops has been uncovered. Prof. Joe Cummins sounds warning of cytokines, to be used in agriculture in place of antibiotics.
Cytokines are small proteins secreted by one animal cell to alter the behavior of itself or another cell. Cytokines send signals to the cell by binding to specific ‘receptors’ on the cell-surface. The biological effects depend on the cytokine and the cell. Typically, these molecules affect cell activation, division, apoptosis (programmed cell death), or movement. Cytokines produced by leucocytes and acting mainly on other white blood cells are called interleukins. Cytokines that have chemo-attractant activity are called chemokines. Those that cause differentiation and proliferation of stem cells are called colony-stimulating factors. Those that interfere with viral replication are called interferons. Interferons protect cells by inducing intracellular production of molecules that interfere with virus replication, and increase recognition of virally infected cells by cytotoxic (cell-killing) T lymphocytes. Interferons also have anti-proliferative effects on some cancer cells.
Cytokines provide useful treatments of infections, and of cancer symptoms and diseases affecting the immune system [1]. Their clinical deployment has been limited by the cost of producing the drug proteins, and recombinant cytokines have allowed them to be used more widely.
In recent years, animal and human genes have been incorporated into crop plants in order to produce vaccines, antibodies, plasma proteins, cytokines and other therapeutics [2], with little thought given to the consequences of the pharmaceuticals genes spreading to food crops or the genes and gene products polluting surface water, groundwater and the air. The first (of possibly many) such disasters has already been uncovered [3, 4].
It has been suggested that recombinant cytokines might provide a safe replacement for antibiotics. Chicken interferon gamma has been proposed as a vaccine adjuvant and growth promoter for chickens [5]. The worldwide production of chickens for meat and eggs is staggering, so the recombinant interferon treatment could spread far and wide in short time. To provide for efficient delivery of the cytokine, adenovirus vectors have been proposed to deliver the cytokine genes to the chicken [6]. The adenovirus has posed significant problems in human gene therapy and is known to cause severe immune reactions including death [7]. The adenovirus may impact the immune system of the treated chicken and also be carried into eggs, offal and meat. Interferon gamma structural gene has a high degree of homology (sequence similarity) to human interferon gamma [8] and a comparison of cytokines in different species shows 6 with 60% or greater homology to the avian gene and tend to cross react immunologically with the protein [9].
As a further complication, the cytokine transcript RNA is subject to alternative splicing to produce different proteins depending on the cellular environment in which the gene is expressed [10]. This will complicate safety assessment considerably.
Chicken interferon has been produced using baculovirus vector in cultured insect cells [11] and in transgenic plants both as a source of the cytokine and as a means of controlling plant disease.
Human interferon alpha has been produced in potato [12] and potatoes expressing interferon alpha were found to resist phytopthora [13]. A ribonuclease gene forming a part of the human interferon alpha virus defense was used to transform potato, the transgenic potato ‘defended’ against virus infection by forming necrotic spots followed by the death of the infected plants 20 days later [14], a rather extreme and impractical form of defense.
However, production of human interferon in crops may provide therapeutic agents for a number of human diseases. Oral ingestion of recombinant human interferon has been reported in over fifty publications involving different disease treatments, such as the prevention of rejection of allograft islet transplants [15]. Cytokine treatments are known to induce sickness and central nervous system toxicity [16,17]. Recombinant human interferon alpha was reported to cause dementia [18], neurotoxicity [19] and mood and cognitive side effects [20].
It is clear that crops expressing interferons would have disastrous consequences as the interferon genes spread and contaminate food crops, and poison our entire food chain. Incorporating them into chickens may well produce demented as well as poisonous chickens.
Other cytokines have begun to be produced in crop plants. Interleukin-10 a powerful immune suppressant used to control graft rejection was produced in open field trials of modified tobacco [21]. I already warned of the potential for ground and surface water to become polluted, and the danger that the transgene product, or the transgene itself could turn a relatively harmless virus into a killer [22]. The human granculocyte-macrophage colony stimulating factor fused with seed glutelin protein was used to create an easy system for oral delivery of the cytokine [23]. Such modified seeds may be widely dispersed by birds and by wind to contaminate food crops or to propagate the modified crop.
Interleukin-2 and interleukin-4 were produced in modified tobacco cells in suspension culture, the cytokines were excreted into the suspension medium allowing for easy recovery and purification [24]. This contained production method should avoid most of the risks of open production in plants in the field [25].
In conclusion, cytokines are proving valuable agents for treating disease, but like vaccines and other drugs, their production should be confined to contained facilities, and field releases of any kind should not be allowed.
Article first published 01/02/03
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