Isotypes and patient-tailored medicine
With so many different strategies to improve the downstream effector
functions of tumour-targeting antibodies, the question arises which
approach to follow. The answer may depend on patient-related factors and
tumour intrinsic characteristics. For example, two FcγR polymorphisms
that affect the binding of IgG antibodies have been described: H131R in
FcγRIIa and V158F in FcγRIIIa. The R131 variant shows lower affinity for
IgG2 while the F158 variant shows lower affinity for IgG1 and IgG3. The
clinical implication of these variants has not yet been fully resolved,
with some studies finding negative correlation with therapeutic efficacy
while others do not60,61. Nevertheless, if larger and
better designed studies confirm the negative correlation between lower
affinity FcRs variants and response to IgG antibody treatment, these
patients may benefit more from IgE, IgA or mAbs optimised for complement
activation. When it comes to complement optimised mAbs it may further be
important to consider tumour microenvironment (TME) factors such as pH
that can affect CDC62 or the expression level of
complement regulatory proteins which allow complement evasion by cancer
cells63. Furthermore, it has been shown that C
reactive protein (CRP) shares its binding site on FcγRs (I and II) and
FcαRI with IgG and IgA, respectively, whereas it can also bind to
C1q64,65. Whether CRP can act as a competitive
inhibitor for FcRs and complement binding of those antibodies in
vivo has not been studied yet, but it could have important
implications. For instance, patients suffering from chronic inflammatory
and neurodegenerative diseases, such as atherosclerosis, type 2 diabetes
mellitus or Parkinson’s disease have chronically elevated CRP
levels66 which may interfere with antibody treatment.
Thus, for those patients, IgE-based antibody treatment might be an
attractive choice. In conclusion, antibody engineering offers a wide
range of opportunities to improve effector functions of mAbs. but
patient-related factors should also be taken into consideration for
optimal isotype selection. This multilevel approach could result in a
more effective personalised treatment.
Antibodies targeting
immunological checkpoint proteins
A recently identified class of mAbs for cancer therapy are the so-called
checkpoint inhibitors. These antibodies do not target the tumour
directly but enhance anti-tumour immune responses by targeting
immunological checkpoint proteins, such as PD-1 or CTLA-4, or their
ligands such as PD-L1. These checkpoint proteins are expressed on
activated T cells and limit excessive T cell responses. As a means of
immune resistance, the ligands of PD-1 are often expressed by tumour
cells67 as well as by myeloid cells infiltrating the
TME68,69. Checkpoint blockade leads to enhanced T cell
activation67,70 and, consequently, the clinical
introduction of checkpoint inhibitors led to a tremendous improvement of
cancer therapy for several different types of cancers.
In theory, checkpoint blocking antibodies do not require Fc-mediated
effects, as their main effector function is expected to be derived from
blocking the receptor-ligand interaction (Fab-mediated). However, it was
found that a functional Fc tail contributed to the therapeutic efficacy
of anti-CTLA4 checkpoint inhibitors in mouse
models71,72. These studies revealed that whereas both
effector T cell (Teff) and regulatory T cell (Treg) populations were
increased in lymph nodes after therapy, within tumours, specifically the
Treg but not the Teff population was decreased. This decrease was only
observed with anti-CTLA4 of the IgG2a isotype (the isotype with highest
A/I ratio in mouse) and appeared mFcγRIV dependent. The underlying
mechanism was found to be caused by a selectively high abundance of
macrophages expressing high levels of FcγRIV in tumours but not in lymph
nodes71. Furthermore, Tregs express much higher levels
of CTLA4 than Teff cells and were therefore preferentially
depleted13. These findings point to the importance of
the TME for therapeutic mAbs efficacy.
There are indications that anti-CTLA4 mAbs show the same effect in
humans. A recent study confirmed the importance of Treg depletion for
human anti-CTLA4 antibody in a hFcγR mouse model73. In
addition, in advanced melanoma patients with high neoepitope burden the
authors found a positive correlation between the presence of the
high-affinity V158 FcγRIII allele and increased response to the CTLA-4
targeting antibody ipilimumab, providing further clinical evidence for
the importance of Fc-mediated function. These findings may be relevant
to explain why only some patients respond to anti-CTLA4 therapy and
provide further rationale to optimise CTLA4 mAbs by improving their A/I
ratio74 or switching to IgA or IgE isotypes given the
microenvironmental requirements for selective tumour Treg depletion are
met.
Similarly, it was shown that the binding of anti-PD-L1 mAb to activating
FcγRs enhances its therapeutic efficacy in mouse models, due to
Fc-mediated depletion of immunosuppressive myeloid cell subsets in the
TME68. However, although another study confirmed that
Fc-mediated depletion of myeloid cells in the TME contributes to the
therapeutic effect of anti-PD-L1 antibodies, this effect was found to be
dependent on the mouse genetic background as it occurred in CT26 tumours
transplanted in BALB/c but not MC38 tumours in C57BL/6
mice69. The depleted myeloid cell subset was the one
with the highest PD-L1 expression whereas PD-L1 expression on the tumour
cells did not contribute to the therapeutic effect of anti-PD-L1
antibody69. Currently, there are three clinically
approved anti-PD-L1 mAbs, two of which have a mutated Fc tail with
abrogated FcγR binding (atezolizumab, durvalumab) and one is a wild-type
IgG1 (avelumab). Since hundreds of clinical trials with these antibodies
are currently ongoing, future results might help to resolve the question
whether a functional Fc tail improves clinical efficacy of PD-L1
targeting antibodies in humans. If so, a further Fc-effector function
optimisation might be an appealing step forward.
In contrast to anti-CTLA4 and anti-PD-L1, a functional Fc tail
compromised the activity of anti-PD-1 mAbs in vivo. The
underlying mechanism of this detrimental effect was the depletion of
tumour-infiltrating CD8+ T cells, which are characterised by high PD-1
expression68. Not surprisingly, two clinically
approved anti-PD-1 mAbs are of the IgG4 subclass, the human IgG subclass
with the lowest A/I ratio. However, since IgG4 still binds to activating
FcγRs to some extent, it would be interesting to compare its therapeutic
efficacy with that of a mutated mAb with completely abolished FcγR
binding75. Similarly, antibodies targeting CD47, a
‘don’t eat me signal’ often upregulated by tumour cells to avoid
elimination by myeloid cells as part of CD47/SIRP-α checkpoint pathway,
do not require Fc-effector function either76.
In conclusion, these findings strongly suggest that the cellular
composition of the TME as well as the relative expression of the target
molecule on different immune cell populations can greatly affect the
outcome of checkpoint blocking mAb therapy. These factors dictate the
need for Fc-mediated mechanisms for an optimal therapeutic effect and,
thus, the isotype selection for checkpoint inhibitors.
(fig.4b ).