By Abby S. Van Voorhees, MD, September 03, 2012
In this month’s Acta Eruditorum column, Physician Editor Abby S. Van Voorhees, MD, talks with Richard L. Gallo, MD, PhD, about his recent Immunity article, “The Antimicrobial Protein REG3A Regulates Keratinocyte Proliferation and Differentiation after Skin Injury.”
Dr. Van Voorhees: Let’s start by setting the stage for your study for our readers. What was known about the role of cytokines in the cutaneous inflammatory response after injury prior to your work?
Dr. Gallo: We have known for many years that there’s a mixture of different types of proteins called cytokines and chemokines that are part of the essential triggers that attract leukocytes to the region to help clean the site of microbes and help facilitate the wound repair response.
Dr. Van Voorhees: Do other organs besides the skin employ similar strategies for protection against infections? Did these other organs suggest looking at the REG3A protein?
Dr. Gallo: The REG3A protein is an interesting molecule. It is best known for its functions in the intestine, where it was discovered as a type of molecule known as an antimicrobial molecule; they serve to control the bacteria that grow at the interface. When REG3A was initially studied it was found to be an intestine-specific antimicrobial. We’ve previously discovered many different kinds of antimicrobials in the skin but in normal skin REG3A was not seen. [pagebreak]
We first wondered whether or not REG3A might be expressed in the skin in situations that aren’t normal. The first idea was to look at psoriasis. Others have previously shown that the skin has a lot of antimicrobials expressed in psoriasis. In fact, larger-scale epidemiologic studies have shown that patients with psoriasis, perhaps because of these antimicrobials, are more resistant to infection. It may even be a survival advantage for the patient with psoriasis.
Dr. Van Voorhees: Tell us about your study. What did you find?
Dr. Gallo: I should mention first that this study was led by Yuping Lai, PhD. It started in my laboratory at the University of California, San Diego. She was in my lab when we made these initial discoveries and started asking what REG3A might be doing. Now she’s an independent investigator in China.
We looked at psoriasis skin samples and were surprised that despite the fact that REG3A was supposed to be an intestinal molecule, the REG3A protein was actually found in very high levels in the skin of patients with psoriasis. We wondered what the role of REG3A was in this situation, since there are many other antimicrobial peptides. We entertained the possibility that it might have a function other than just killing bacteria.
The next step was to do some simple experiments and just watch what happens when REG3A is placed with normal skin cells. We were surprised to see that REG3A by itself would cause keratinocytes to grow more quickly. So it looked like rather than being just an antimicrobial, it was a growth factor. That was interesting and the question was asked: Is this something that’s really involved in the pathophysiology of psoriasis? Is this important to explaining why there’s a hyper-proliferation of skin characteristic of psoriasis? As you know, psoriasis is a disease where when normal skin is scratched the skin can Koebnerize; we see excess proliferation after subtle injury. So we looked at normal wounds and saw that REG3A is also a part of normal wound response. The key breakthrough came when Dr. Lai discovered that a molecule that clinical dermatologists have now become very familiar with, IL-17, was involved in controlling REG3A production. [pagebreak]
To digress a bit, we’ve learned in dermatology that biologic therapy can be extremely effective at treating psoriasis. The drugs that treat the TNF pathway have been very effective, and more recently we find that drugs that act on the IL-17 pathway, another critical step in inflammation, can also be very effective. But neither drug really explained the pathophysiology of the disease because it wasn’t clear how either IL-17 related proteins or TNF-related proteins would cause keratinocytes to grow more quickly, the finding we see in psoriatic plaques. Our study found that REG3A is a critical intermediary between IL-17 and skin proliferation.
Dr. Van Voorhees: How is REG3A thought to exert its effect?
Dr. Gallo: What we think is happening is that when there is an excess amount of IL-17, which has a lot of important roles in the immune system besides causing skin proliferation, the IL-17 in the skin is stimulating too much production of REG3A. The REG3A is then recognized by a receptor that Dr. Lai also identified, known as EXTL3, and REG3A binds to the EXTL3 receptor on the cell surface of keratinocytes and then they proliferate. So when we improve patients with an anti-IL-17 therapy, we’re knocking out everything IL-17 can do, including knocking down the induction of REG3A. Now we’ve got a handle on something more downstream, and even more focused, that may specifically target the inflammatory and proliferative response in psoriasis. We were able to prove that in a mouse model of psoriasis where we stimulate production of more IL-17 but can block proliferation and development of psoriatic lesions with an antibody to REG3A. [pagebreak]
Dr. Van Voorhees: Let’s talk a little bit more about what you’re seeing in psoriasis skin. There’s an increase in REG3A in lesional skin in psoriasis but not in other inflammatory conditions like atopic dermatitis. Why do you think that may be true?
Dr. Gallo: The simple answer is we’re not sure yet but we believe it reflects the different kinds of cytokines one finds in psoriasis versus atopic dermatitis. Most importantly, psoriasis is more of an IL-17-predominant disease; there’s a lot of IL-17 which as I mentioned induces REG3A, whereas in atopic dermatitis, a lot of the TH2 cytokines like IL-4 and IL-13 are more abundant rather than IL-17, and they do not stimulate REG3A.
Dr. Van Voorhees: In your paper it states that in some populations you saw evidence of REG3A in both normal and lesional skin, whereas in others you really only saw REG3A in the abnormal psoriatic skin and not in non-lesional skin. Why do you think that might be?
Dr. Gallo: We’re still puzzled by that. We did the study on a number of different psoriatic populations, including patients from the United States and Japan, and we did see a difference in the normal skin between those two groups. The patients in Japan seemed to have more REG3A even in the normal skin. There could be a lot of different explanations for that, including different treatment modalities and where they were in their disease progression. We’re still working on trying to understand that. [pagebreak]
Dr. Van Voorhees: Did you primarily obtain your samples from patients with plaque psoriasis? Did you have opportunities to look at other types of psoriasis?
Dr. Gallo: We did focus on plaque psoriasis. We don’t have enough data yet to see if there’s a difference in pustular, guttate, erythrodermic, or other kinds of psoriasis.
Dr. Van Voorhees: How about the other clinical scenario you looked at. What you have shown in wounds?
Dr. Gallo: The wounds recapitulate the response we saw in psoriasis, but they have a normal regulatory mechanism, which we don’t understand yet, that shuts the process down once the wound is repaired. So shortly after injury of the skin there’s an infiltration of macrophages and monocytes and they start producing some of the T cells that are recruited and are producing IL-17, and they produce REG3A in the keratinocytes at the wound edge and it helps them proliferate more. We think in a wound environment, REG3A is very important for normal wound repair. [pagebreak]
Dr. Van Voorhees: So why do you think you did not see REG3A in the disregulation we see in squamous cell carcinoma?
Dr. Gallo: The cytokine milieu in those kinds of inflammatory situations, much like the difference between psoriasis and atopic dermatitis, is not appropriate for stimulating REG3A.
Dr. Van Voorhees: Do you feel that this allows for a possible target that could be explored to treat psoriasis?
Dr. Gallo: Absolutely. It raises the question if in situations where there’s abnormal wound repair, if there’s a possibility that REG3A might help kickstart and accelerate the process. That’s been tried with many other growth factors in the past; it’s a complicated system so it’s worth a try but not as obviously a pathway to success.
On the other hand, we’ve had such great response with biologic therapy in psoriasis that we’re actually quite optimistic that a targeted approach of inhibiting REG3A might be a way to focus down and have less of the broad immunosuppressive effects of targeting TNF or IL-17 pathways and more focus on one of the end molecules that cause the manifestations of the disease of the skin, the REG3A itself. [pagebreak]
Dr. Van Voorhees: Does its size make the development of topical treatment possible? Patients would be very excited to have a truly efficacious topical biologic treatment for severe psoriasis. Since this disregulation is in the epidermis, if a treatment could penetrate that would be an interesting way to go about treating it.
Dr. Gallo: It’s a pretty big protein. Absolutely that could happen. We have two candidate targets for treatment here. One is to neutralize the REG3A and the other is to block the receptor the REG3A is binding to, the EXTL3. So small molecules that could penetrate the skin could possibly be effective in this way, and not rely on the antibody-neutralization approaches that have typically been used in the anti-TNF or anti-IL-17 pathway drugs.
Dr. Gallo is professor and chief of dermatology at the University of California, San Diego and chief of dermatology at the VA San Diego Healthcare System. His article was published in Immunity, Vol. 37, 111. doi: 10.1016/j.immuni.2012.04.010.