Cish is a NK cell checkpoint inhibitor

This week several science news outlets spotlighted new study from Nature Immunology showing anti-tumor effect of Cish deficient NK cells. This study showed that Cish deficient NK cells are hyper-responsive to its canonical cytokine IL-15 and show improved control of experimental tumors.

I analysed this study to determine if the buzz was deserving. In my view this study is in fact two independent [and not connected] studies put together artificially. The finding that Cish deficient NK cells have superior anti-tumor behavior is based on non-physiological experimental model.

First part of this study deals with cytokine sensitivity of Cish deficient NK cells. This is exclusively in vitro experiments. It does show that Cish deficient NK cells are hyper-responsive to NK cell canonical cytokines such as IL-15, or IL-12/IL-18 combo [and other activatory receptors].

Second part of this study, however, is completely dissociated with IL-15 story and jumps directly to tumor protection experiments with WT or Cish deficient NK cells. These experiments showed that Cish deficient mice are resistant to exogenously injected tumor cells and this protection depended on asiolo-GM1+ cells and IFN-γ.

Finally, the authors showed that adoptive transfer of Cish deficient NK cells into NK-deficient hosts (Ncr1^Mcl1Δ/Δ mice) also provided protection against i.v. injected melanoma cell metastasis.

In summary, this study proposed that Cish functions as a checkpoint inhibitor for NK cells.

My view:

(A) It is not clear whether Cish deficient NK cells alone is sufficient for anti-tumor effect [it is independent of CD8 T cells, but CD4 T cells involvement was not tested].

(B) It is not clear what role cytokines such as IL-15 or IL-12/18 play in vivo in Cish deficient mice.  

(C) One of the major differences between NK and T cells is that NK cell effector functions are not regulated in an antigen-specific manner but rather by sensing signaling balance between activatory and inhibitory membrane receptors [functions like a rheostat].

During ontogeny individual NK cell undergoes its own "adjustment" to its environment and can even "tolerate" naturally arising tumor cells. This is why experimental tumor models when tumors are injected exogenously, and appear in the body out of the blue so to speak, do not recapitulate natural interaction with NK cells and easily could produce biased, non-physiological response from first-time encountering NK cells. The more physiological tumor models will be to use spontaneously arising mouse tumor models.

David Usharauli

    

CD4 T cells provide universal "help" to CD8 T cells via pathogen-tailored DCs

Ordinarily [but not always] naive CD8 T cells require "help" from CD4 T cells to undergo full differentiation and to develop into memory. Such CD4 T cell help is provided via so called "licensed" antigen-presenting cells, DCs. In a simple scenario, when pathogen invades tissue, local DCs will pick up its antigens and present them to both CD4 and CD8 T cells. In turn, activated antigen-specific CD4 T cell "licenses" the same DCs to up-regulate or secrete necessary molecules to complete priming of naive CD8 T cells (I am going to use terms "help and "license" interchangeably).

This simple model is complicated by fact those "licensing" molecules for CD8 T cells differ depending on pathogens. The most well described "helps" include IL-12, IL-15 or type I IFNs. So, how CD4 T cells are able to deliver so many different licensing signals?  

Apparently, CD4 T cells don't. According to new paper published in Cell Reports, CD4 T cells simply amplifies pre-existing pathogen-tailored signals within DCs. Lets see if data are convincing (note, this paper was under review process for > 2 years)

Initially, the authors confirmed that CD8 T cell priming/expansion during viral infection, HSV-1, required presence of CD4 T cells, MHC II, CD40L or CD40.

Next, the authors showed that CD8 T cells priming/expansion during HSV-1 infection required signaling via either IFNαR or IL-15.

Experiments with BM chimeras, IL-15^KO:CD11c^DTR and IFNαR^KO:CD11c^DTR, revealed that DCs-specific expression of IFNαR and IL-15 were required for CD8 T cell priming during HSV-1 infection.

However, production of IL-15 by DCs in response to IFNα also required presence of CD4 T cells.

In fact, ex vivo stimulation of CD8α⁺ DCs with IFNα and αCD40-mimetic (as a surrogate for CD4 T cell help) showed that CD4 T cell "help" amplified IL-15 induced by innate [viral-induced] IFNα (since αCD40-mimetic alone had no effect). However, it is not clear whether αCD40-mimetic could fully recapitulate CD4 T cell function. So, this requires additional tests.

Dominant role of innate signaling in determining the nature of CD4 T cell "help" was revealed in experiments in which mice were challenged with cell-associated OVA in combination with LPS or Poly(I:C). In presence of LPS, "help" was IL-12 dependent, while in presence of Poly(I:C), "help" was IL-15 dependent.

In summary, the conclusion of this study, according to the authors, is that CD4 T cells simply amplify pathogen-tailored innate signals already generated within DCs, rather than proving unique maturation signals. My interpretation of these results is not very different from earlier models. I don't think that anyone claimed that CD4 T cell "help" and innate signals were completely interchangeable. For me, "licensing" and in this case "amplification" are very same concepts. For me, more important question is how those CD4 T cells that deliver "help" are getting activated in first place (basically, who primes the "primers").

David Usharauli

Hair follicles attract normal and lymphoma T cells via common gamma chain (γc) cytokines IL-15 and IL-7

Skin is a body's second largest surface that is continuously exposed to exogenous antigens. So, no wonder that it attracts and harbors specialized immune cells. One of these immune cells are skin resident memory T cells (T_RM cells) who play an important role in local skin epidermal defense as a rapid deployment tactical units. Besides this protective function however, T_RM cells can contribute to pathological conditions such a drug allergy or lymphomas.

New paper just published in Nature Medicine showed that accumulation of normal or oncogene-transformed T_RM cells in skin epidermal layer is directed by hair follicle derived IL-15 and IL-7.

First, the authors confirmed that T_RM cells are present in both skin dermal and epidermal layers (but CD8 T cells were present only in epidermal layer).

Next, the authors generated RAG-deficient BM chimera where the host's non-hematopoietic tissue [such as keratinocytes] also lacked IL-15 expression. Transfer of wild-type T cells into these IL-15/RAG double-deficient host showed that non-hematopoietic cell-derived IL-15 was important for CD8 T cell [but not CD4 T cell] recruitment to the epidermis [though it is not clear why would the authors suggest that non-hematopoietic tissue-specific IL-15 deficiency equals to hair follicle-specific IL-15 deficiency].

Further experiments with mice with keratinocyte-specific conditional IL-7 deletion revealed that cytokine IL-7 also played important role in recruitment of both CD4 and CD8 T cells to the skin [again, the authors have used this model as an example of hair follicle-specific IL-7 deletion. Not sure how accurate is this notion].

Accordingly, the authors showed that skin contact hypersensitivity response to hapten DNFB that requires T cell activation were reduced in mice with non-hematopoietic tissue-specific IL-15 and IL-7 deficiency.

Finally, using mouse T cell lymphoma model, that mimics human cutaneous T cell lymphoma (CTCL), the authors showed that keratinocytes-derived IL-7 was important to direct accumulation of T lymphoma cells to the epidermal layer.

In summary, this study provided mechanistic explanation for some of the clinical features of human T cell lymphomas. It appears that keratinocytes [hair follicle]-derived IL-15 and IL-7 actively attract both normal and cancerous T cells to the epidermis. Since both IL-15 and IL-7 are common gamma chain (γc) cytokines, inhibition of their signaling with JAK3 inhibitors [such as Pfizer's JAK1/3 inhibitor Tofacitinib] may provide some relieve in pathological situations such as drug [hapten]-induced allergy and T cell lymphomas (CTLC, mycosis fungoides, Sezary's syndrome).

David Usharauli