The immune system is a powerful protective part of human homeostasis. Invaders, external and internal, can be identified, tagged, attacked and disposed of. On the other hand, the immune cells often take part in protecting and enhancing the viability of such invaders as cancer cells. We examine this dual role of elements of the immune system based on several approaches to the current understandings.
Several of the elements of the immune system we focus upon are the macrophages, the
neutrophils and the mast cells.
They are all part of the innate immune system and all are in a sense part of
the first barrier of defense to intruders of homeostasis. We reflect upon these as
below.
Tumor activate macrophages (TAMs) have been known as cells
which have protected and supported tumor cells. TAMs have been examined by many
researchers and there seems to be a slow but advancing opportunity to address
them and eliminate their influence[1]. We now summarize with several observations relating to
putative extensions of what we have presented above.
There appears to be a putative multiplicity of targets for
therapeutic applications. As Pathria et al have recently stated:
TAMs express cytokines
and enzymes that
can suppress T cell recruitment and activation, thereby promoting resistance to
immune checkpoint inhibition. Bone-marrow-derived and tissue-resident TAMs each
contribute to TAM overall content, and both can promote tumor immunosuppression. In preclinical mouse
models, inhibitory targeting of myeloid cell surface receptors (PD-L1,
CD47/SIRP1α, CCR2, CSF1R, and integrin α4β1), signaling components (PI3Kγ, mTORC1, BTK, and PDE5),
transcription factors (KLF6, STAT3, TWIST, ZEB1, and NFAT1), metabolic pathways
(arginine metabolism), and others, can prevent tumor immunosuppression and
synergize with immune checkpoint inhibitors to improve antitumor responses.
Epigenetic regulation
of macrophage polarization – as with Class IIa HDAC
inhibitors – may protect from cancer immunosuppression by stimulating macrophage
proinflammatory gene expression, and thus activating cytotoxic T cell antitumor
responses. Antagonists of several targets, including CSF1R, CCR2, CD47/SIRP1a,
PI3Kγ, BTK, and HDACs, as well as agonists of TLRs are currently under clinical
investigation as putative cancer therapies for various malignancies.
Macrophages are phagocytes that serve as a first line of defense against
pathogenic insults to tissues. These innate immune cells mount proinflammatory
responses to pathogens and repair damaged tissues.
However, tumor-associated macrophages (TAMs) express
cytokines and chemokines that can suppress antitumor immunity and promote tumor
progression. Preclinical studies have identified crucial pathways regulating
the recruitment, polarization, and metabolism of TAMs during tumor progression.
Moreover, novel
therapeutics targeting these pathways can indirectly stimulate cytotoxic T cell
activation and recruitment, and synergize with checkpoint inhibitors,
chemotherapy and/or radiation therapy in preclinical studies. Thus, clinical
trials with therapeutic agents that promote phagocytosis or suppress survival,
proliferation, trafficking, or polarization of TAMs are currently underway. These
early results offer the promise of improved cancer outcomes.
Also, as we
have seen certain growth factors exacerbate the development of metastatic
processes. VEGF is one. In a paper by Holash et al they had
noted:
Vascular endothelial growth factor (VEGF) plays a critical role
during normal embryonic angiogenesis and also in the pathological angiogenesis that
occurs in a number of diseases, including cancer. Initial attempts to
block VEGF by using a humanized monoclonal antibody are beginning to show
promise in human cancer patients, underscoring the importance of optimizing
VEGF blockade. Previous studies have found that one of the most effective ways
to block the VEGF-signaling pathway is to prevent VEGF from binding to its normal
receptors by administering decoy-soluble receptors.
The highest-affinity VEGF blocker described to date is a
soluble decoy receptor created by fusing the first three Ig domains of VEGF
receptor 1 to an Ig constant region; however, this fusion protein has very poor
in vivo pharmacokinetic properties. By determining the requirements to maintain
high affinity while extending in vivo half-life, we were able to engineer a
very potent high-affinity VEGF blocker that has markedly enhanced
pharmacokinetic properties. This VEGF-Trap effectively suppresses tumor growth and vascularization in vivo, resulting
in stunted and almost completely avascular tumors.
VEGF-Trap-mediated blockade may be superior to that
achieved by other agents, such as monoclonal antibodies targeted against the
VEGF receptor.
We have noted this previously based on other studies. As
with many immune control mechanisms one must always look at the downside and
controlling VEGF broadly can have significant morbidity. However, if one can
have multiple markers then perhaps specific targeting is possible.
Recently Ngambenjawong et al have reported:
With deeper understanding of cancer immunology, diverse
strategies for modulation of TAMs are being uncovered and explored for
therapeutic applications. Due to the complexity of tumors, combination therapy
is typically needed to maximize an anti-tumor response.
Thus, a clear understanding of the modes of drug action
as well as mechanisms of resistance is needed in order to design an efficacious
combination therapy that minimizes antagonistic effects. For example,
identification of PI3k up-regulation in tumor as a resistance mechanism for
CSF-1R kinase inhibitor in recurrent glioma suggests that a combination therapy
between PI3k and CSF-1R inhibitors could be more beneficial. Conversely, the
therapeutics aiming to block macrophage recruitment signals (e.g. CCL2 or
CXCL12 inhibition) may not be compatible with the ones that require the presence
of macrophages for anti-tumor actions (e.g. anti-CD40 antibody).
In congruence with the well appreciated immunosuppressive
roles of TAMs, a consensus was observed regarding the potential benefits of
TAM-targeted therapies in potentiating immune checkpoint blockade therapies
(anti PD-1/PD-L1/CTLA-4 antibodies) as evidenced in the race among
pharmaceutical companies to investigate such combination therapies (Table 2).
Improvement in gene sequencing and analysis technologies greatly facilitates
the adoption of precision medicine where patients could be examined for genetic
makeup and matched with appropriate therapeutic regimens.
Documenting patients’ genetic profile and the
corresponding therapeutic outcome are also beneficial in correlation studies to
better predict patient response as well as in refining drug development.
In the case of CSF-1R inhibition therapy, certain single
nucleotide polymorphisms (SNPs) in CSF-1R have been identified that reduce the
potency of emactuzumab. Nonetheless, the study may help in the future design of
the next-generation CSF-1R blockade therapy. To reap the benefit of a steady
rise in molecularly targeted therapies that are promising for clinical
translation, it is more than ever important to be resource-efficient. This may
be possible through careful validation of pre-clinical studies and innovative
design of clinical trials as seen, for example, in the I-SPY 2 trial
(NCT01042379).
The above comment regarding SNP variability and interference
draws the issue regarding inter-patient genetic variability. However one
suspects that sequencing the genome is not necessarily the answer. The SNP
variability may be more prevalent than thought but hidden perhaps by epigenetic
factors such as methylation of histone compression. They continue:
With numerous therapeutic targets being identified and
drug candidates being explored for modulation of TAMs, drug delivery
technologies will soon come into play to further enhance therapeutic efficacy
of these drugs, for example, by improving pharmacokinetics, stability,
selectivity, or intracellular delivery while limiting systemic toxicity.
Together with advancement in gene-editing technology,
effective silencing of genes that promote pro-tumoral functions of TAMs (e.g. STAT3,
SIRPα, PI3k, or Gpr132) may one day be a practicable therapeutic option.
Finally, a cost barrier is another factor that could impede the clinical
translation especially when multiple antibodies are used in a combination
therapy as currently investigated in several trials.
The problem may be alleviated with improvement in
manufacturing efficiency or development of cheaper alternatives such as small
molecule drugs or peptide analogs.
Indeed, the potential for a pan-tumor-environment
therapeutic regime has potential but only after a better and complete
understanding of the TME. Furthermore the plasticity of the TME may become a
challenge itself.
There is an ongoing need for non-invasive markers for
various malignancies. Most of these markers would be blood borne such as PSA
albeit with greater specificity and sensitivity. One generally tries to obtain
AOC (area under the curve) performance as high as possible. From Bilen et al we
have such a discussion regarding the markers available from immune counts as
follows:
Optimal prognostic and predictive biomarkers for patients
with advanced-stage cancer patients who received immunotherapy (IO) are lacking. Inflammatory markers, such
as the neutrophil-to-lymphocyte ratio (NLR), the monocyte-to-lymphocyte ratio
(MLR), and the platelet-to-lymphocyte ratio (PLR), are readily available.
The authors investigated the association between these markers
and clinical outcomes of patients with advanced-stage cancer who received IO. …
Baseline and early changes in NLR, MLR, and PLR values
were strongly associated with clinical outcomes in patients who received
IO-based treatment regimens on phase 1 trials. Confirmation in a homogenous
patient population treated on late-stage trials or outside of trial settings is
warranted. These values may warrant consideration for inclusion when risk
stratifying patients enrolled onto phase 1 clinical trials of IO agents. …
Elevated baseline and early increases in NLR, MLR, and PLR
values are strongly associated with poor clinical outcomes in this cohort of
patients treated on IO-based phase 1 clinical trials. These values may warrant
consideration when risk-stratifying patients who are being enrolled onto phase
1 clinical trials. Given the high concordance among these variables, any of
these markers may be used in future analysis. Future studies investigating the
relation between changes in these values and the tumor microenvironment are crucial to elucidate the
underlying biologic explanation for the prognostic and predictive value of
these markers.
Generally, the pathological analysis of tumors addresses the
malignant cells with minimal recognition of the presence of the various immune
cells and other elements of the ECM. The question then is; can we
use the information on the tumor associated immune (TAI) cells to further
diagnose and stage the lesion? We have addressed this question obliquely
elsewhere when discussing just what we mean by cancers[1].
Namely, there are certain lesions where the cells are
clearly aberrant yet they are not proliferating and the immune ECM involvement
portends a benign progression. Thus perhaps adding details on path reports so
that a larger study may be performed would have value.
Macrophages such as the M1 and M2 are not necessarily fixed
but exhibit great plasticity, moving from one extreme to another. As De Palma
and Lewis have noted[2]:
Once resident in tissues, macrophages acquire a distinct,
tissue-specific phenotype in response to signals present within individual
microenvironments. The exact combination of such tissue-specific cues dictates
both the differentiation and activation status of these cells.
Two extreme forms of the latter are generally referred to
as “classical” (or M1) and “alternative” (or M2) activation, which parallel
Th1/Th2 programming of adaptive immune cells.
During acute inflammation, macrophages are M1-activated
by toll-like receptor (TLR) agonists and Th1 cytokines (e.g., interferon
[IFN]-γ). This enhances their ability to kill and phagocytose pathogens,
upregulate proinflammatory cytokines (e.g., interleukin [IL]-1β, IL-12, and
tumor necrosis factor-α [TNF-α]) and reactive molecular species, and present antigens
via major histocompatibility complex (MHC) class II molecules.
Alternatively, Th2 cytokines, like IL-4 and 13, stimulate
monocytes/macrophages to express an M2 activation state. This is characterized
by higher production of the anti-inflammatory cytokine, IL-10; lower expression
of proinflammatory cytokines; amplification of metabolic pathways that can
suppress adaptive immune responses; and the upregulation of cell-surface
scavenger receptors, such as mannose receptor (MRC1/CD206) and hemoglobin/aptoglobin
scavenger receptor (CD163).
As such, M2 macrophage activation may facilitate the
resolution of inflammation and promote tissue repair (including angiogenesis)
after the acute inflammatory phase. In healthy tissues, macrophages often
express a mixed M1/M2 phenotype; hence “M1” and “M2” polarization should be
regarded as extreme ends of a continuum of activation states, with their exact
point on the scale depending on the precise mix of local signals present in a
given microenvironment.
Thus the M1 state is the attack and kill state and the M2
state is the protect and grow state of the macrophage colony. The M2 state does
this perforce of the plasticity of the macrophage which initially sees the
"invader" and attacks and then sees the result and tries to
"heal" the wound. In doing the latter is in effect facilitates the
cancer cells growth. Perhaps, therefore, turning back from M2 to M1 would allow
the attack to continue towards some resolution. Yet as know, there can be a
multiplicity of unintended consequences.
Some of the plasticity effectors and processes are shown
below based upon the work of Noy and Pollard.
Understanding the role of TAMs, and the TME, we can see that
in vitro models suffer greatly due to the lack of the putative protective and
plastic environment surrounding cancer cells. The challenge to those doing such
work is to reflect the true holistic environment of the tumor. De Palma and
Lewis have noted:
Mouse tumor models, including genetically engineered
mouse models (GEMMs), are being used extensively to study mechanisms underlying
tumor (and TAM) responses to anticancer therapies. However, even sophisticated
GEMMs of cancer cannot simulate the endless variations in TAM abundance,
distribution, and phenotypes between and within different types and subtypes of
human cancer. Nor do they necessarily model the ability of such tissues to
recruit monocytes during therapy.
Future work should therefore aim to define the identities
and molecular profiles of distinct TAM subtypes in human cancer biopsies
before, during, and after therapy. Specific TAM signatures could then be used
to stratify patients carrying defined genetic lesions in order to explore how
such signatures correlate with the response of individual patients to chemo-,
radio-, or targeted therapies, and/or the emergence of secondary resistance. If
such studies demonstrate the predictive value of specific TAM subtypes for
individual tumor responses, then their further characterization in mouse tumor
models could help develop more effective cancer therapies. Undeniably, such
clinical approaches should consider the biological complexities on a tumor
(sub)type and individual patient basis and harness them to design effective
personalized therapies.
Indeed, one must be cautious regarding the ability to
project from one controlled environment to the human species. Perhaps that is
one reason why we all too often see but a 30-40% response rate to many
immunotherapeutic approaches, namely the complex variability and plasticity of
the TME.
References
- Pathria et al, Targeting Tumor-Associated Macrophages in Cancer, Trends in Immunology, March 2019
- Holash et al, VEGF-Trap: A VEGF blocker with potent antitumor effects, PNAS August 20, 2002 vol. 99 no. 17 11393–11398
- Ngambenjawong et al, Progress in tumor-associated macrophage (TAM)-targeted therapeutics, Adv Drug Deliv Rev. 2017 May 15; 114: 206–221
- Bilen et al, The prognostic and predictive impact of inflammatory biomarkers in patients who have advanced-stage cancer treated with immunotherapy, Cancer. 2019 Jan 1;125(1):127-134
- De Palma and Lewis, Macrophage Regulation of Tumor Responses to Anticancer Therapies, Cancer Cell, March 18, 2013
Posts
