Anti-CD73 molecule CPI-006 activates B cell responses to SARS-CoV-2

As the COVID-19 pandemic continues to cause an immense impact across many parts of the world, the role played by immune responses is becoming more evident every day. A recent funded paper published on the preprint server medRxiv* in September 2020 reports the initiation of a Phase I trial of a drug that increases antibody and cell-mediated immune responses against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

The neutralizing power of antibodies in COVID-19 is related to their ability to inhibit virus attachment to the host cells at the angiotensin-converting enzyme 2 (ACE2) receptor. The higher the titer of neutralizing antibody, therefore, the more effective they are at clearing the virus and promoting clinical recovery, as well as containing viral spread.

Cell surface enzyme CD73 is being studied as a target for cancer immunotherapy. It has been found earlier to be involved in the trafficking of lymphocytes and the activation of T cells. However, CD73 may also play a part in the maturation of B cells, based on studies in patients with common variable immunodeficiency (CVID), who cannot produce Immunoglobulin G (IgG).


Most human B cells display the CD73 receptor involved in the activation of lymphocytes and their subsequent migration to a site of inflammation or infection. So does a subset of T cells. This molecule also has an enzyme function, catalyzing the conversion of AMP to adenosine. This change can modulate the immune response, in contrast to its other pro-inflammatory effects.

The current paper discusses CPI-006, an anti-CD73 monoclonal IgG1 antibody (mAb) that does not bind FcgR. It inhibits the enzymatic function of CD73 but activates B cells that express it.

The researchers say, “These immune effects suggested that CPI-006 may be effective at enhancing the magnitude, diversity, and duration of humoral and cellular responses to viruses such as SARS-CoV-2.”

Examination of the B cell receptor (BCR) at the molecular level in patients treated with CPI-006 indicates that novel B cell clones are both produced and expanded by this molecule. On this basis, the researchers sought to find out if this could be developed as a drug for COVID-19 immunotherapy, resulting in higher antiviral immune responses. Preliminary findings from the trial show that it can be useful in treating COVID-19 as well as used in combination with vaccines to induce immunity to the virus.  

CPI-006 Suppresses CD73 Enzymatic Function

The researchers found that this molecule the enzyme-mediated immunosuppressive function of CD73. Using human peripheral blood mononuclear cells (PBMCs), cultured in conditions that favor T cell activation, they found that when AMP was added, T cells did not proliferate as vigorously as before, and cytokine secretion was also reduced. In the presence of CPI-006, this reverted to normal, by preventing the AMP-to-adenosine conversion.

The same results were seen with interferon-gamma (IFNg) production, which reflected the T cell response to this molecule.

CPI-006 is a Unique B Cell Activator in Vitro

Many human blood cell lineages in the body express CD73, which is thought to promote the activation and adhesion of lymphocytes. The researchers screened the immune cells for surface markers that were differentially expressed after treating with CPI-006, using flow cytometry.

They found that CPI-006 dramatically induces B cell activation in a dose-dependent manner. In fact, at concentrations of 1 μg/mL, the activation marker CD69 was expressed at near-maximum levels.

B cell antibody production increased antigen presentation markers such as CD86 and MHC-II, and B cell maturation was also observed. These activated B cells also transform into plasmablasts, or antibody-secreting cells. Another mechanism of B cell activation is via the canonical signaling pathways in the body.

Such phenomena have not been described by earlier researchers concerning the CD73 signaling pathway and may be seen only with CPI-006. Moreover, CD69 expression is enhanced only by the bivalent binding of CPI-006.

Phase 1 CPI-006 Trial in Advanced Cancer

A phase I trial currently underway in advanced cancer patients is evaluating the role of CPI-006 for immunotherapy. The results indicate that at all doses of CPI-006, the circulating CD73POS B cell frequency drops steeply, within half an hour of treatment. On the other hand, this molecule does not cause B cell death or antibody-dependent cytotoxicity, which indicates the drop in B cells is due to their redistribution after activation, into lymphoid tissues. This is a transient change, and within three weeks, these cells re-entered the bloodstream at levels comparable to the baseline but enriched in class-switched memory B cells.

Diversified BCR Repertoire

Memory B cells are immune antibody-producing cells where immunoglobulins have been rearranged in order to generate antibodies with a high affinity to specific antigens. In some patients, 2-40 novel B cell clones were found to emerge, at frequencies up to 1 in 100 B cells, indicating an antigen-specific substantial clonal expansion. It has been shown that this is consistent with unchanged antigen-specific IgG responses to five common viruses, such as measles and mumps viruses, in the third week after treatment. This response is not, therefore, merely a polyclonal activation of B cells but rather a specific response to an antigen.

Safety Profile

The findings of this trial of different doses of CPI-006 in patients with a median age of 64, with other illnesses including diabetes and obesity, show no adverse effects. Universal recovery was observed, with discharge at a median of 4 days after hospitalization.


The addition of CPI-006 led to a threefold rise in IgM and IgG1 levels compared to a control, indicating both increased antibody formation and perhaps class switching as well. Cytokines such as CCL3, CCL4, CCL2, and CCL22 are also found to be at higher levels following CPI-006 treatment. But this did not increase the levels of the pro-inflammatory cytokines IFNg, IL-2, IL-6, IL-10, or TNFa. This is an important consideration in COVID-19, whose severity is thought to be linked to a hyper-inflammatory state.

A steep surge in the titer of IgM and IgG antibodies against either SARS-CoV-2 or the RBD or both was seen to occur within a week of a single low dose of CPI-006. While the duration since the first symptom is not related to the antibody levels at the study baseline, CPI-006 induced a rise in IgM to over 100,000 in one patient, which persisted until 28 days later. This correlated with an increase in neutralizing antibody levels and is expected, from the results of earlier studies, to correspond to those of plaque reduction neutralization tests.

IgM and IgG levels continue to rise, even at 56 days in one patient. And these exceeded antibody titers in convalescent sera even when these came from patients who would have been expected to have better antibody responses.

Examination of PBMCs shows that memory B cells are increased and memory and effector CD4POS and CD8POS T cells. The cytokine profile indicates it is skewed towards a Th1 response.


The researchers explain, “CPI-006 induces the expression of CD69, an activation marker, resulting in the prolonged retention of activated B cells in lymphoid organs. This increased lymphoid residence time provides time to complete B cell activation and interact with CD4POS T follicular helper cells to shape downstream immune responses.

Neutralizing anti-TS and anti-RBD antibody titers rise within a week of administration of one low dose of CPI-006. This dose is above the 1 μg/ml threshold at which B cells activation is highest, but is low enough to permit it to leave the body rapidly. In addition to the intravenous route used here, subcutaneous and intramuscular administration also become potential realities in this case.

In addition, further research will be needed to understand how these B and T cells perform against the virus and whether they can help regulate both types of immunity in a balanced manner. If indeed CPI-006 achieves long-term sustained elevations of the antibody response, it could be useful not only in treating COVID-19 but also in adjuvanted vaccines for increased titer, diversity, and duration of antibody production as well as its effect on memory cells.

The diversification of the BCR could mean that the virus cannot easily escape the effects of the resulting immune activation by escape mutations. It would also help with developing more mAbs from the isolation and sequencing of specific cells.

The researchers note the need for a randomized controlled trial to prove that/ if these antibody enhancing effects are truly due to CPI-006. Yet, they say, “These encouraging early results are in line with our hypothesized biological mechanism and we remain cautiously optimistic.”

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Dr. Liji Thomas

Written by

Dr. Liji Thomas

Dr. Liji Thomas is an OB-GYN, who graduated from the Government Medical College, University of Calicut, Kerala, in 2001. Liji practiced as a full-time consultant in obstetrics/gynecology in a private hospital for a few years following her graduation. She has counseled hundreds of patients facing issues from pregnancy-related problems and infertility, and has been in charge of over 2,000 deliveries, striving always to achieve a normal delivery rather than operative.


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