Interviewer: How did you get onto thinking about the thymus?

Lymphocytic leukaemia induced by Gross virus began in the thymus and then spread elsewhere. Many other types of lymphocytic leukaemia in mice do exactly the same thing: you can induce leukaemia in low-leukaemic strains by various agents, such as irradiation or chemical carcinogens like dimethylbenzanthracene. All these lymphocytic leukaemias begin in the thymus and then spread. In fact, the spontaneous leukaemia of high-leukaemic strain mice, which are called AK, also begins in the thymus. In America and other places it had already been shown that thymectomy (removal of the thymus) in older mice would prevent both spontaneous leukaemia and leukaemia that had been induced by the various agents I mentioned, provided it was done at about 1 or 2 months of age – so preventing leukaemia from starting at 9 months of age, as it normally would.

That had not yet been done for the virus-induced leukaemia, so one of the first things I did was to see if thymectomy prevented virus-induced leukaemia. I injected the virus at birth and then thymectomised the mice when they were a month old, and it worked – it prevented leukaemia. The next question was obvious: put the thymus back with a graft and see if leukaemia begins again in the thymus. And it did. That was to be expected, but what was not expected was that when I put the thymus back 6 months after adult thymectomy, thinking that by then surely leukaemia could not start again, it did. That suggested that the virus must have remained latent for all that time.

The next experiment was simply to thymectomise adult mice that had been inoculated at birth with the virus, wait for 6 months – not give them a thymus graft – take their tissues, make an extract of their normal tissues and see if that induced leukaemia. And it did. So that was the logical outcome of the experiment.

If it showed that the virus was generalised throughout the body, not localised to the thymus, was the thymus the critical place where it multiplied, presumably because the lymphocytes there were the ideal cells in which to replicate and produce leukaemia?

Probably that is totally incorrect but it is exactly what I thought. You would have to give the virus at birth because the thymus at that particular stage was producing very large lymphocytes – people did not know then why it produced lymphocytes – and perhaps those lymphocytes were the ideal medium for virus replication. So if you inject the virus at birth, it goes to the thymus, replicates and then migrates to the rest of the body. My idea was that if you thymectomised first, at birth, and then injected the virus immediately afterward, leukaemia would not occur – that was obvious – but when you gave the thymus back later on, as I had done, leukaemia should still not occur because the virus would not have had a chance to multiply.

I can see that. That’s what my experiment would have been.

So I had to learn the technique of neonatal thymectomy. I could do it in the adult but it is a bit different in the newborn. Finally I worked it out, and actually it’s quite easy.

I remember meeting you at Pollards Wood in 1962 and learning from you how to do a neonatal thymectomy. I never used it in my subsequent work, though.

The one problem about neonatal thymectomy is cannibalism by the mothers, so I had to do a lot of mice. I must thank my wife: she helped me very nicely by making sure that the mothers would not eat the babies after thymectomy, or changing them around until she found some that did not eat babies.

Why did the mice waste away? An unexpected finding

I became worried because although all the mice that survived the operation were perfectly healthy with their mothers and until after weaning, about 4 to 5 weeks of age, they then suddenly wasted and became sick. This was never recorded after adult thymectomy – you can take the thymus of an adult mouse and it lives perfectly well into old age – but after neonatal thymectomy, generally at 6 to 8 weeks depending on the strain, they started wasting away and many of them died. I couldn’t understand that.

I did a post mortem, of course, and I found several things. One was lesions in the liver, which looked as if they were infected by a virus – hepatitis virus, for example. But also I found a deficiency of lymphocytes in the lymph nodes and spleen, which was quite remarkable. At that time Jim Gowans and Peter Medawar had both shown that lymphocytes in the spleen, in the lymph nodes and in the recirculation pool – that is the lymph and the blood – were the cells which could initiate immune responses. (They were called immunologically competent cells.) My mice had a deficiency of those lymphocytes, which led me to think they must surely come from the thymus.

Yes, because the thymus is filled with lymphocytes.

Correct, but in those days it was thought that because the thymus had a lot of dying lymphocytes it was a graveyard for them. Not only that, but thymus lymphocytes were not able to induce immune responses in appropriate recipients as the lymphocytes from the spleen and lymph nodes or the recirculating pool were. And also because adult thymectomy did not interfere with immune capacity, people did not believe that the thymus had anything to do with immunity.

Yet, because of the results that I had obtained, I really wondered whether the thymus was the seat of production of those lymphocytes which would eventually become competent. Maybe they were just immature in the thymus – they had to mature and go out, and then become competent. I wrote that idea up in the Lancet in 1961, after being pushed to write it by Sir Alexander Haddow, who was officially my mentor.

Focus questions

• Miller used mice as his experimental animals. What might have been the reasons for his choice?

• Miller mentions that chemical carcinogens will induce leukaemia, a type of cancer. Can you suggest some other cancers that have been shown to be induced by chemical carcinogens?